Based on analysis about anti-missile fight flow of shipborne gun guiding shell,the outer trajectory of guiding shell was divided into three phases: preliminary phase,guided phase and shell-target encounter phase.All of shooting error headstream in each phase was analysed,and the main makeup of shooting error was nailed down.The numerical character of sub-error and its method to be ascertained was put forward,and its predigested technique in numerical simulation was discussed.The research result would avail the weapon system efficiency simulation in next research.