Why didn't Song Ziwen want to sign the Sino-Soviet Treaty 1945?

Because the treaty has two major problems. First, Outer Mongolia was allowed to leave China by referendum. In fact, outer Mongolia is controlled by the Soviet Union. The referendum is that the Mongolian people controlled by the Soviet Union voted to leave China. Second, some treaties require the Soviet Union's privileges in northeast China: China and the Soviet Union have been in charge of Changchun Railway for 30 years, and Lushun enjoys a naval base for 30 years. The country is shameless, whoever signs it will be scolded for hundreds of years. Song Ziwen is not stupid, of course he won't sign it.