Abstract: Based on category description for selfnetwork externalities of B2B platform, this paper builds a twosided market theoretical mode, and focuses on whether a B2B monopoly platform has selfnetwork externality or not, which selfnetwork externalities of bilateral users is more strong, and reveals the internal mechanism of selfnetwork externalities for B2B monopoly platform. The studies show that in B2B monopoly platform, the selfnetwork externalities from platform bilateral user’s has always been brought about a negative effect. Both the competition selfnetwork externality and the demonstration selfnetwork externality of the seller are stronger than that of the buyer. Combined with theoretical analysis, it puts forward some competitive strategies for B2B platform, such as improving service level, expanding demonstration selfnetwork externality, and pricing obliquely.