Abstract: In this paper, the equations of FEL amplifier with aplanar wiggler are derived. FBC code based on equations is designed by H.P.Freund. WAGFEL code based on KMR theory are designed by us. Although there are obvious differences between WAGFEL and FBC code, in dealing with practical problems such as magnetic field and length of the entry wiggler, initial distribution of electron beam, fashon of the downstream tapered wiggler etc, a set of c-oupled nonlinear differential equations used in two codes is based on single-particle theory. Meanwhile, under the certain conditions, the computational results of both WAGFEL and FBC code coincide with those of the experiment well. Then the computional results used by WAGFEL and FBC code are compared.The results of TE₀₁ mode calculated with WAGFEL and FBC code are compared and analyzed under the condition of the constant and varyable amplitude of wiggler magnetic field B_w, respectively. The analyzed results show that regard the conditions and results of experiment as criterion, WAGFEL code is more useful in comparing similation results with experiment than FBC code. As a result, not only data of 4MeV amplifier which are computed by WAGFEL code are valuable, but WAGFEL code is feasible to design amplifier and study problems of FEL.