Abstract: Because of some practical reasons, we suppose that the claim number of a given policy holder is a negative binominal distribution and the structure function of the portfolio is a Beta distribution, then we get a mixed negative binominal distribution(i.e. negative binominal-Beta distribution). The paper discuss the property of the mixed negative binominal distribution and its application to a set of practical data from insurance practice. The result shows that the mixed negative binominal distribution can fit the real observation of claim numbers very well. So we use this model to construct the optimal BMS to adjust the renewal premium of the policy holders. Compared with other BMS’s, this one is more feasible in practice.