Determination of the basic probability assignment (BPA) is the first and main step to the evidence theory application. How to generate BPA is still an open issue. To solve the problem, a method for determining BPA based on the cloud model is proposed. Firstly, the normal cloud model of each sample under the property is constructed based on the backward cloud generator. Secondly, through the antecedent cloud generator repeatedly, the average certainty of the test sample under this property is obtained. Thirdly, a method for measuring the similarity of normal cloud models is proposed, and the maximal similarity of the normal cloud model is made, which has the maximal certainty as the belief of the universal set. Finally, the certainty is normalized to obtain the BPA of each class. The effectiveness of the method is proved by experiments, and it can generate BPA in the case of little samples numbers.