Based on the reality in China，three kinds of market competition models of network video platforms，symmetric free (FF) ，asymmetric payment (PF) and symmetric payment (PP) ，are constructed in this paper． The evolutionary mechanism of the business modes of network video platforms are revealed by analyzing and comparing the three models，and the role of the positive and negative cross-network externalities of platforms in the evolutionary mechanism of the platform business modes is analyzed． The results show that if certain differences exist between platforms，the equilibrium results and business strategies are affected by the positive and negative cross-network externalities． With the change of the positive and negative cross-networks externalities，the business modes of network video platforms will undergo three stages of evolution: “symmetric free mode”，“asymmetric payment mode”and “symmetric payment mode”． Correspondingly，the business strategy adopted by the platform will also undergo four stages: “free + advertising”，“subsidy + advertising”， “membership fee + advertising”and “pure membership fee”． However，only the FF and PP modes are the Nash equilibrium results of the network video platform competition，while the asymmetric competition (PF) could not reach the Nash equilibrium．