Theoretical analysis and numerical solution of several physical mechanisms on nondegenerate four-wave mixing(NDFWM) in semiconductor laser amplifiers are presented. Starting from the general carrier rate equation, density-matrix equations and nonlinear wave equations, considering the effects of carrier-density pulsations, carrier heating, and spectral-hole burning, we obtained three sets of nonlinear coupled wave equations, and compared the contributions of the three different physical mechanisms to the NDFWM. We also investigated the effect of linewidth enhancement factor on the conversion efficiency of the NDFWM in semiconductor laser amplifiers.