Aiming to the disadvantage of computational complex and long time-consuming of traditional Monte-Carlo method, establish guided missile target relative motion equation using the covariance analysis describing function technique (CADET). The principle of CADET is introduced in the paper. Then study the linearization of nonlinear function, analyze the application of CADET in proportional guidance control system. And give an example to analysis it. The results show that the method has the advantage of higher speed, statistical performance and precision in the guidance performance analysis. It is better than Monte-Carlo method.