The change of social system in former Soviet Union was an extremelyinfluential and complicated event in socialist movement history. Quite differentviews have been raised in the past 10 years about its causes, among them are "utopians", "original sins", "peaceful changes", "national conflicts","high level autonomy","failure in economy",""the fault of Stalin model","wrong line", "ideology","abandon", "synthetic forces of history", etc. Of these views, only "utopian" and "original sin" are obviously prejudiced while other views more or less explored the causes from different fields, layers and viewpoints,and contain reasonable elements. But undeniably in our country scholars have beentaking the study of the change in Soviet Union as a caution to our reform andcentral work. Naturally such studies rest merely on superficial level, not deep-going. In fact, the fundamental cause for the social system change in former Soviet Unionwas due neither to the fault of Stalin model nor to failure in economy nor to wrongreform line nor to national conflicts. The change took place not because theproletarian class and people of Soviet Union gave up the Party but because thebenefit-gained stratum within the Party headed by Gorbachev consciously acted insuch a way. Social system change in former Soviet Union was caused by theconscious promotion of the benefit-gained stratum within the Party.