Perceived trust is one of the most important prerequisites for consumers to make trading decisions. Taobao.com provides an evaluation system for sellers to judge a seller’s degree of trust in terms of its historical transaction. However, according to prospect theory, consumers’ judgment was warped in some cases. Taking the evaluation system of taobao.com in China as an example, it analyzed decision makers' psychological activities when they facing positive and negative evaluations. Meanwhile, In order to quantitatively evaluate consumers’ perceived trust, it recalculated consumers’ perceived probability about sellers’ positive evaluation and negative evaluation based on prospect theory. The result shows, consumers’ perceived negative evaluations are not the same as the negative evaluations listed on website. In fact, consumers usually overstate sellers’ negative evaluation, and adjust their perceived trust. E-commerce sellers should be vigilant even if their negative evaluation probability were quite low.