On the basis of the physical model of a four-level neodymium-doped phosphate waveguide amplifier, the rate - transfer equation of a neodymium-doped phosphate waveguide amplifier in which the pump light at the wavelength of 802nm is used to amplify the signals at the wavelength of 1060nm is studied by using an overlap integral method. The plot of the gain against the waveguide doping concentration, the pump power and the waveguide length are obtained through numerical simulation.