Theoretical studies on laser gain of a multi-stage dye laser amplifier are presented in this paper. The results show that the influences of amplified spontaneous emission (ASE) and nonlinear absorption of excitedstate on the gain are different for different input laser energies and gains. A threshold input intensity exists for a specific system. If the input intensity is higher than the threshold, the nonlinear absorption of excited-state will be the main cause for gain decrease. Otherwise, the ASE is the main cause. A new scheme is proposed to calculate the gain of the amplifier by the comparison of the input intensity with the effective saturation intensity and the choice of the calculation gain method.