【摘要】Because of some practical reasons, we suppose that the claim number of a given policyholder is a negative binomial distribution and the structure function of the portfolio is a Beta distribution, then we get a mixed negative binomial distribution(i.e.negative binomial\|Beta distribution).The paper discusses the property of the mixed negative binomial distribution and its application to a set of practical data from insurance practice. The result shows that the mixed negative binomial 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 policyholders. Compared with other BMS's, this one is more feasible in practice.