Abstract: Due to the deficiency of online transaction contracts, the lack of credit reporting system and the imperfection of law, public order could not solve the problem concerning credit in online transaction and the private order concerning reputation has gained wide attention recently. According to the principle of “incentives, coordination and authority”, the private order is analyzed into three kinds: “the spontaneous”, “the intermediary” and “the organized”. An online transaction model is established to theoretically prove the role of private order in preventing moral risk in online transaction. Therefore, according to the three kinds of private order, it is necessary to design and improve external mechanisms to guarantee that the private order plays a stable role in the place of public order to some extent.