A United States-China relationship which had gone sour is once again becoming sweet and sour. This was the impression left by the American side as Secretary of State George Shultz ended a second round of talks here on Feb. 3. The thorny issue of Taiwan was discussed by Mr. Shultz and China's Foreign Minister Wu Xueqian.
But Chinese unhappiness over American support for Taiwan was not being allowed to prevent progress on other issues. The fact that Shultz was scheduled to see the Chinese defense minister, Zhang Aiping, was seen as a signal that China may still be interested in buying some form of military equipment from the United States.
In a statement to American businessmen here on Feb. 3, Shultz said: ''I'm very pleased with the manner in which our talks with the Chinese are proceeding this week. They are serious, constructive, and wide-ranging. With good will, cooperation, and a sense of purpose that both sides bring to the cause of advancing the relationship, there is much we can and will accomplish - and at least in my judgment, the future bodes well for the United States and China relationship.''
This seemed to indicate that the talks here were going just about the way Shultz wanted them to go. John Hughes, the State Department spokesman, said that the talks had not only been serious and constructive but also ''friendly.''
The American side indicated nonetheless that there were still a number of areas of disagreement with the Chinese. These included Taiwan, differing approaches to some third-world issues, and the way in which the US has been handling trade with China, and the sale of high technology to this nation.
But the Chinese did not appear to dwell on these points. They stated their position to Shultz and then moved on to other issues. Mr. Hughes said that Taiwan, the most difficult of the issues, took up only about 20 to 30 minutes among several hours of talks between Secretary Shultz and Foreign Minister Wu.
According to Hughes, Wu asked for a clarification of policy on the technology sales issue. The Chinese have complained that the American government is slow to grant approval to many of their requests for such sales.
Shultz also heard complaints on the technology question from American businessmen based in China. Asked by one businessman why European and Japanese governments were quicker to grant licenses for exports to China than was the US government, Shultz grew short-tempered.
''Maybe they are just better,'' Shultz replied. ''What don't you move to Japan or Western Europe?''
Shultz explained that in the administration of export controls there are ''sometimes difficult issues of interpretation and that tends to prolong consideration of given items.''
But the secretary said that progress had been made in that approvals were increasing and the rate of pending cases decreasing.
In his statement to the businessmen, Shultz said that the US fully supported the Chinese in their modernization goals.
''A stable, secure, economically-healthy China, participating actively and constructively in the mainstream of the international economic system is in the best interests of the United States, of the East Asian region, and of world peace,'' said Shultz.
He also made this plea to the businessmen: ''Let us not get so bogged down in bilateral arguments so as not to see and understand how far we have come in so brief a period of time. China is now the 14th largest trading partner of the United States . . . also the fourth largest market for American agricultural products.''