Abstract: For the rotating mortar ammunition,strapdown homing guidance system based on the math platform is designed.Firstly,equivalent rotating vector algorithm is applied to calculate the attitude transfer matrices between the body coordinates and the inertial coordinates,restraining the noncommutativity error. H<SUB<∞</SUB< square root filter,using the information measured by the strapdown seeker,and is then designed to estimate the relative movement variables between the ammunition and target.Optimal proportional guidance law,using the estimated information,is finally used to guide the ammunition.Simulation about the nonlinear aerodynamic model of rotating ammunition showed that the strapdown homing guidance system could attack the maneuvering target efficiently.