Considering the performance of short-time working, inertial flight and high impulse of launch for projectile, a spherical simplex UDUT unscented Kalman filter (SS-UDUTUKF) is proposed to estimate the state of attitude determination system. According to the special motional rule of projectile, only use three low-cost MEMS accelerometers as inertial measurement unit to measure the specific force, and construct the linear state equation and nonlinear measurement equation of system. Use the UDUT UKF to estimate the state of nonlinear system, which effectively enhances the stability of filter and restrains the divergence of filter. The SS-UDUTUKF algorithm has good numerical stability, avoids losing the non-negative definiteness and symmetry of estimation covariance and effectively restrains the divergence of filter resultingly from rounding error of calculation. The experimental results in a three-axis flight test rotary table show that the proposed algorithm obtains the good precision of estimation and satisfies the request of real-time response of projectile's attitude determination.