Taipei, Taiwan — In Taiwan's military headquarters, a Chinese scroll hangs on a wall. It reads: ''Be as pure as jade.'' The sign expresses a taintless virtue especially valued these days among Taiwan's military leaders.
A feeling of righteous indignation exists here over the increasing sales of United States technology to Taiwan's enemy - the mainland communists - while the Reagan administration slowly reduces sales of US military hardware to Taiwan, a former American ally.
''We get little help from the US,'' said Gen. Sheng Wang, recent head of the military's political warfare department and a leading candidate to become president of the Republic of China (Taiwan), perhaps next year.
''We can't even buy all the military equipment we need, yet we stand here like a guard for the US.''
In mid-July, the Reagan administration announced a $530 million sale of military equipment to Taiwan, part of a total sale of $800 million expected this year, mainly for surface-to-air missiles and parts for tanks and aircraft. Peking believes this exceeds the amount allowed under the 1982 Sino-US Shanghai communique, which calls for a phasing down of US military sales to Taiwan.
Taiwan's defense brass say they are working against time, a time when US military sales end - and when Peking will give up its attempt to peacefully reunify Taiwan with the mainland and launch a military offensive.
American officials agree Peking is ''scared.'' But they say much of the US-China friction over Taiwan is really posturing for the day when China and Taiwan try to strike a deal for reunification. (Taiwan watches like a hawk the present British-China talks on the future of Hong Kong.) What worries Peking is the possibility that Taiwan may someday show no interest in reunification.
This could occur as the majority population of native Taiwanese (who have never seen the mainland) eventually take over the leadership from the minority of exiled mainlanders, who now control almost all key posts.
This process of ''Taiwanization'' began slowly a decade ago. With Chiang Ching-kuo (son of the late leader Chiang Kai-shek) possibly stepping down in 1984, more native Taiwanese will rise up the ranks - although Ching's replacement will likely be a mainlander.
Reunification, however, will always be in the heart of any Chinese, says government spokesman James C. Y. Soong. 'It's in the sentiments of the culture. We look at it like the Middle Kingdom. We believe there is one sun over the earth, one government. It's part of our history,'' he says.
Taiwan's military chief believes Peking's real threat is ''the prosperity on this island next to poverty on the mainland. When the people on the mainland begin to rebel, that is the real threat,'' says Mr. Soong.
He fears that the ''modernization'' program under way in the People's Republic of China (PRC) really means military modernization. ''It increases the ability of the mainland to take over the island. It is not only a threat to Asia , but to the world.
''At present, we can maintain the military balance over the Formosa Strait (between Taiwan and the mainland). But in the years ahead, our enemy will have developed the F-8 (fighter jet). They will have a production line by the mid-80 s. Then we might become vulnerable.''
At present, about 390 combat aircraft defend this small island nation of 18 million against the mainland's estimated 5,000 fighter jets. Taiwan's request to maintain a qualitative edge by buying high-performance American aircraft (the F- 5G or F-16) has been denied. US officials contend Taiwan can counter the mainland with ''technical upgrading'' and good use of present weapons.
''We have to find other sources (than the US) for a sophisticated fighter, or develop it ourselves. We are working to have the capability to make such a fighter by the late 1980s. We are designing the craft right now to fly at Mach-2 in all weather,'' Mr. Soong said.
But US officials say Taiwan does not have the capability to develop a sophisticated aircraft.