SInce traditional effectiveness evaluation method of GPS jamming is based on static analysis, measure of effectiveness and conclusion of evaluation are too hard to understand and can't reveal rules of jamming. In this paper, a new evaluation methodology of GPS jamming based on combat simulation was proposed. Firstly, the distance between bomb droppoint and target was selected as measure of effectiveness; Secondly, mathematical models were established based on theories of jamming and aerodynamics; Thirdly, GPS jamming effectiveness was valuated through simulating the trajectory of SINS/GPS guided bomb moving after GPS jamming; Finally, the conclusion of GPS jammer deployment was got based on simulation result.