The theoretical model of Er-doped waveguide amplifier with longitudinal step-dopand was build.The finite element method,rate equations and propagation equations were used to simulate numerically net gain characteristics of the amplifier. The results show that pumping efficiency of the amplifier is improved. Net gain and signal output power of step-dopant waveguide amplifier increase 9.2% and 90.5% respectively than uniform-dopant waveguide amplifier. Length of the amplifier,however,is shortened 16.9%.