Peking — With the opening of the 12th Communist Party congress in Peking, Deng Xiaoping is on the verge of putting his stamp on the future course of China.
The congress, which opened Sept. 1, will endorse Mr. Deng's flexible economic policies and approve new leadership arrangements favorable to the dominant Dengist faction.
The congress is also expected to open the way for an extensive ''rectification'' campaign among the Communist Party's 39 million members. The size of the party is likely to be reduced.
Thus, this series of meetings is shaping up as one of the most important in the party's history. Deng no doubt hopes that the policies to be endorsed will carry through to the end of this century and that China will finally escape the series of debilitating internal party struggles that have characterized its post-1949 history.
In an opening speech to the congress, Deng said his three main tasks for the 1980s were to intensify the pace of modernization, to strive for reunification with Taiwan, the island province still ruled by the Nationalist Chinese, and to fight hegemonism.
Deng and his supporters have been maneuvering for several years toward an extensive ''weeding out'' of those considered by the Dengist moderate faction to be unsuitable for membership.
According to a well-placed Chinese source, party members, following the Congress, will be required to re-register for membership. Acceptance of re-registration will not be automatic.
Chinese officials are insisting the campaign will not turn into a purge. Lu Dingyi, director of the Central Committee's propaganda department, wrote in a People's Daily article last week that the campaign would be conducted on the basis of ''proper criticism and self-criticism.''
Deng has said that many party members are ''not up to standard.'' He was referring particularly to that group, numbering perhaps more than 20 million, who joined the party since the beginning of the Cultural Revolution.
The organizational changes to be endorsed by the Congress are also an important part of Deng's strategy. The positions of chairman and vice-chairmen will be scrapped. A central advisory panel of party elders, expected to be headed by Deng, will be set up. And the discipline inspection commission of the party is expected to be strengthened along with the party Secretariat headed by Hu Yaobang, the present party chairman.
The scrapping of the positions of chairman and vice-chairmen (there are six vice-chairmen) appears a shrewd move by Deng. In the public mind, the chairmanship has always been associated with Mao Tse-tung, and it has therefore been difficult for the rather nondescript Mr. Hu, a Deng protege, to make much of an impact in the position.
Western observers here believe Hu will be more secure as secretary-general, effectively the party's chief executive, and a position, incidentally, that he was appointed to in 1980 and did not relinquish at the time he took on the chairmanship in 1981.
The abolition of the chairmanship is being sold to the party rank and file as part of the move toward collective leadership, something the Dengists have been emphasizing since the end of 1978.
The eventual shape of the Politburo and its standing committee remain a matter for speculation, but it seems certain that with an enlarged role for the Secretariat, headed by the party's chief executive and the establishment of a panel of senior advisers, that the Politburo's role will be diminished.
Another important task facing the Congress is to endorse a new party constitution. A draft of the constitution has not yet been published, but it is certain to be less ideological in content than the existing one that rests heavily on the need to pursue class struggle - an exhortation now seen by the Dengists for the most part as a waste of time.
The 1,600 delegates at the congress will, of course, elect a new Central Committee, which is expected to include an overwhelming number of Deng's supporters, unlike the more ambivalent present Central Committee, which was elected in 1977 at a time when Maoists like Hua Guofeng, the former chairman, were in the ascendancy.
In a report to the present congress, Mr. Hu indirectly criticized Mr. Hua for the erroneous policies of the Cultural Revolution. The party has been through a good deal of trauma in the past several decades and millions of Chinese have suffered because of what some say arecapricious policies pursued under the weight of ideological struggles in the 1960s and early 1970s.