Alliances are one of the most effective ways for states to preserve and gain power. The study of alliance formation has long been a prominent issue in international politics. This article examines the factors that influenced the alliance between Sun Yatsen’s regime and the Soviet government through the lens of alliance theory, taking the “alliance with Russia” of the Guangdong government led by Sun Yatsen in the early 20th century as the subject of study. The scientific innovation lies in the analysis of historical events and their causes from the perspective of international relations. Due to the complexities of the Chinese situation, the southern regime under Sun Yatsen coexisted with the Beijing government, but was not recognized as the official government of China. As a result, Sun Yatsen needed to use an “alliance approach” to seek the backing of external forces in order to keep his own regime secure. After several failed  attempts to enter into alliances with Britain, the United States and Germany, Sun Yatsen eventually elected to join forces with the Soviet Russia. According to the alliance’s conclusion, four major factors influenced Sun Yatsen’s “alliance with Russia” policy: balance of threat, diplomatic assistance, ideology, and political infiltration. Among them, balance of threat was the main motivation for Sun Yatsen’s alliance with Russia, and while ideology was not an important factor influencing the alliance, the ability to provide assistance to Sun Yatsen’s regime played an important role in the establishment of the alliance, considering the huge power gap between the two sides. The Soviet political penetration of Sun Yatsen’s regime helped the alliance’s establishment, but also led to its final dissolution.