Abstract: By constructing the structure function of claim size lognormal distribution parameter y, an optimal estimator of the policy’s future claim size was elicited.An empirical rating actuarial model with allowance for claim frequency and claim severity was presented. Thus, the pricing on automobile insurance is more equitable and reasonable. Furthermore, a typical rating actuarial model fit for Chinese automobile insurance reality was put forward，preliminary to the promotion of rates allowance for claim severity.