Fair exchange protocols are different from classical security protocols, which resulted in the differences in design principles between them. Therefore, when designing fair exchange protocol, the designer needs to reconsider some other factors. Based on the characteristics of fair exchange protocols, in this paper, the authors put forward four new design principles. In order to illuminate the importance of these principles, the authors give some attack examples. With these principles, the designer can consider the various possible vulnerabilities and errors at the beginning of fair exchange protocol design. Therefore, the quality of protocol design is improved greatly.