Guided projectile usually adopts inflight alignment as power-on after launching to obtain initial navigation information, among them, the roll angle in the initial attitude information is more difficult to obtain than the pitch and the yaw. The article proposes a combined navigation solution suitable for large initial roll angle error to solve the difficulty of obtaining the roll angle information in the in-flight alignment, it can omit the roll angle alignment and shorten the inflight alignment period. Introducing quaternion error to the combined navigation state vector and converting strong-nonlinear model of the state equation into second-order nonlinear model can decrease the complexity of the system. On this basis, the second-order extended Kalman equation is used to estimate each error of the inertial navigation system and using quaternion mathematical model 1 constraints can get an optimized measurement update method. A hardware-in-loop simulation result for each order of magnitude of the initial roll angle error shows that this method is suitable for large initial roll angle error, the roll angle error can be converged from 180° to less than 10° within 7 s, and the convergence position is stable and consistent for each order of magnitude of the initial roll angle, and it can effectively improve the adaption of the combined navigation.