To improve the firing accuracy of small caliber naval gun, this paper studies the model of its fire control system in case of P56-type system (JM90). Based on the principles of P56 type fire control system, firstly the more general and more accurate shooting parameter solving algorithm for P56 is derived; secondly the manual installing target velocity and slant range parameter are put forward. At last, discuss problems which limit the validity of ship born P56 fire control system. The research results show that use the method, the manual installing parameters reduce from 2 to 1, and the modified quantity is determined by the previous shooting miss distance to determine artificial setting, and also reduce the subjectivity of manual installation parameter.