Based on the essential characteristics of two-sided markets, this paper analyzes the role of competition between users within platforms in the competition between platforms from three aspects: user scale effect, platform stickiness effect and platform competition effect. We found that in the most realistic oligopoly market structure, the effect of platform competition plays a leading role, and competition between users within platforms intensifies competition between platforms. Because ranking competition is a unique competition mode in internet platforms, this paper further investigates the role of ranking positions on platforms’ multiple types of competition. Top-ranked merchants have a strong platform competition effect due to flows and sales volume, which has a positive effect on competition between platforms. However, this effect weakens with the backward-ranking of positions and leads to an overall negative effect. Finally, this study conducts a heterogeneity analysis of different platform modes and shows that the internal market concentration of self-operating e-commerce is significantly more concentrated and that the effect of merchant competition on platform competition is weaker than that of platform e-commerce.