In order to avoid the blindness of battlefield command for military officials, it was necessary to pre-estimate the average ammunition consumption according to operational mission. Aiming at the characteristic of ship-borne shrapnel, simplified approach to firing error was analyzed. Based on basic principle of uniform distribution method, the evaluation formula of damage probability was put forward. By inverse operation of damage probability, the computational model on average ammunition was put forward. The results show that the computational model holds the advantages of simple formula and fast calculation, and it provides scientific evidence to establish decision-making during firing on shore.