The accurate estimation of the initial error and the guidance instrumentation error plays a key role in the precision assessment of the maneuvering launched vehicle, so a new estimation method based on nonlinear model was proposed. The platform initial angle error was transformed into the orientation error. The true apparent acceleration was accurately calculated by using the fixed point iteration method. Then, the trajectory parameters of the truth launch coordinate were represented by a nonlinear function with the initial error and the guidance instrumentation error. A nonlinear model was constructed by using the exterior trajectory measurements. This model can simultaneously estimate the initial error, the guidance instrumentation error, the measurement systematic error, and the time-zero deviation between telemetry data and exterior data, and it can avoid the linear approximation of the initial error. The Bayes MAP (maximum a posterior) estimation is given to obtain the optimal estimation of these errors by using the nonlinear model and the prior information, and it is proved to be convergent. Experimental results show that the proposed method improves the estimation accuracy of the initial error and the guidance instrumentation error when compared with the linear method and other nonlinear method. Furthermore, the proposed method can also achieve the selfcalibration between different measurements.