The United Nations and UN Secretary-General Kurt Waldheim have become the unwitting victims of a significant hardening in China's US policy. Offended by the Reagan administration's approach toward Taiwan, the Chinese have reacted sharply - and Mr. Waldheim has been caught in the middle.
For two months, the Chinese have blocked Mr. Waldheim, who is the choice of the majority of the Security Council (including the US), in his bid to be reappointed as secretary-general. of the UN.
UN sources say the Chinese government has been angered by Reagan's de facto ''two-China policy,'' and his refusal to rule out the sale of advanced fighter planes to Taiwan. China's objections to the such sales were conveyed to United States Secretary of State Alexander M. Haig Jr. in September by China's vice-foreign minister, Zhang Wenjin, and were dismissed by the White House.
After one more frustrating encounter between China's Foreign Minister Huang Hua and President Reagan on Oct. 26, China, these same sources say, decided to pull back sharply back from its partnership with the US and resume its strident pro-third-world stance of the early 1970s.
Indeed, for the last four weeks, Peking has issued a number of official communiques describing both superpowers once again as equally ''evil.'' In recent years, the Soviet Union had been singled out as the main villain. Thus China's growing disenchantment with the Reagan administration culminated in a reassessment of its relations with the US. Peking's consistent veto of Mr. Waldheim has created a two-month impasse at the Security Council.
By withdrawing his name from further balloting, Mr. Waldheim has broken the deadlock. But no solution is in sight, and the UN may have been badly damaged.
This view is shared by many senior diplomats here.
''Indeed,'' says one Asian diplomat, ''it is ironic. China and the third world need the UN more than the industrial nations.''
High-ranking Western officials feel Waldheim and the UN are the victims of Reagan's China policy and, in the words of one European expert on Asia, of ''his ideological hangup on Taiwan and on Chiang Kai-shek's heritage.''
The effect of China's hardening vis-a-vis the US was felt at once at the UN. On Oct. 28 the Chinese mission received ''unexpected and surprisingly tough'' new instructions from Peking.
Instead of supporting Tanzania's Salim A. Salim against Waldheim with only two successive vetoes, as had been anticipated, the Chinese went all out and vetoed Waldheim's reappointment 16 times.
Moreover, Salim's approval by the Organization of African Unity and the nonaligned movement - was obtained not through democratic consultation but strong-arm tactics.
''One of the ironies of the present situation where countries like Spain, France, Ireland, and China have courted Salim, is that he never enjoyed solid third-world support,'' says a respected African ambassador.
Finally, a small band of hard-liners has been criticized for having lost sight entirely of the UN Charter by forgetting that the secretary-general is not supposed to be imposed on the membership by a bloc of nations, but to be chosen by the Security Council exclusively on the basis of his personal merits and of his political acceptability to the Council members.
Salim is now expected to withdraw from the race, since a US veto will continue to block him indefinitely and he will not be able to blame his defeat on Waldheim any longer. Presumably, several Latin American candidates will then be given their chance. They, as well as Prince Sadruddin Aga Khan, former UN high commissioner for refugees, are expected to be humbled by either a Soviet or a US veto.
Then, come Dec. 31, China will be confronted with a hard choice:
* Either forget its vows to stick it out with the third world, and agree to reappoint Waldheim for only two or three years, with an ''understanding'' that his successor would be from the third world.
* Or imperil the organization by further blocking any move by the Council to request Waldheim to stay in office and either leaving the UN without a leader or replacing Waldheim with one of his aides.