We propose a new method to discuss the evolution of physical systems, which has an analytic form. And we systematically introduce this new method by the example of Jaynes-Cummings model without rotation wave approximation. Simultaneously, while we repeat previous work by our method, we also calculate it by adding the time growth factor to the initial state unfolded in the steady state which is based on Fock state and is obtained by solving the time-independent Schrodinger equation (in this article, the traditional method refers to this method). By comparing these two results, we find the drawback of our method and improve it. Finally, we show that our improved method need the smaller Fock space than the traditional method for physical systems with two-mode cavity field and put forward expectations for the follow-up study.