On the basis of defining the core concepts of Internet financial platform and platform monopoly power, the paper systematically sorts out the formation path, evolution mechanism and social welfare loss measurement method of Internet financial platform monopoly. Combined with panel data, it examines Chinese Internet financial platform by stages and types. Whether there is a monopoly, the social welfare loss that may be caused is measured, and the relationship between the monopoly of the Internet financial platform and the social welfare loss is verified. The research results show that: Chinese Internet financial platform monopoly is generally at a relatively high level, and the monopoly strength of different stages and types of Internet financial platforms is significantly different; the resulting social welfare losses have the characteristics of large scale and obvious structural differences; There is a relatively significant one-way mechanism of social welfare loss, that is, monopoly causes social welfare loss, but the latter does not necessarily aggravate monopoly in the opposite direction.