How to survive and grow has always been a serious problem faced by the small and medium sized enterprises (SMEs) on a B2B platform. A lot of researches in entrepreneurship field have proved the positive effects of entrepreneurial orientation on firms' performance, but few researchers studied how entrepreneurial orientation influences performance. Based on the institutional theory, this paper introduces legitimacy and reputation to try to explain how competitive aggressiveness affects SMEs' performance in a B2B environment. Based on the analysis of data from 400 SMEs on a B2B platform, several interesting findings are concluded. Competitive aggressiveness has a very significant positive influence on SMEs' performance. It is more important to firms on a B2B platform than traditional firms. Both legitimacy and reputation plays a moderator role in this process, the better corporate legitimacy and reputation are, the stronger this influence is.