Why was the Soviet Union boycotting the Security Council at the time of the Korean conflict?The Rise and Interaction of Superpowers 1945-1962

The reason that the Soviet Union was boycotting the United
Nations Security Council meetings at this time had to do with the issue of
China.

In 1949, the Chinese Communists had won the civil
war that they were fighting against the Kuomintang nationalists.  The Kuomintang had
then fled to the island of Taiwan, where they set up their own
government.

At that point, both the mainland, communist
Chinese and the Nationalists on Taiwan claimed to be the real government of China.  The
UN was recognizing the Nationalists at this point and so the USSR
boycotted.