Through discussing the evaluation methods of subjective trust in open network, the fuzzy quality of subjective trust was investigated by introducing credibility theory, and the quantitative evaluation approach of subjective trust was presented based on the criteria of maximizing expected value. Then under the situation that the key attributes of subjects and the weight of evaluation were characterized as fuzzy variables, a simulating method was given to calculate the expected value of subjective trust. Finally, the presented approach was proved to be feasible and effective by an example.