Abstract: Aim at the security issuses in cloud computing, a method based on Combination Weighting Approach of Relative Entropy is proposed to evaluate user's behavior trust to analyze the untrust and abnormal behavior effectively,so as to achieve the scientific quantitative evalution for user's behavior trust.Combined with the Weighting factor of weighting method which is both subjective and objective,relative entropy is applied to compute the approach degree of weights.Which are set by subjective and objective weighting method, thus the weighting efficient of each weighting menthod's weight in conmbination weight can be determined.Finally,fuzzy comprehensive assessment is made for combination weight to get user's trust value. Implementation and verification of the model shows that combination weighting method and the model are scientific and operability,and making objective evaluation for the user's credit.