Master oscillator power-amplifier (MOPA) configuration was widely used in high-power laser system. In order to find the optimal ratio of oscillator to amplifier, influences of this ratio on the near-field intensity distribution and the output power were studied. Fast Fourier transform method was used to calculate the near-field intensity distribution and the powers at the ratios of 1:4, 1:1 and 2:1. The simulation results indicated that when the total volume of the gain is constant, the output power of the oscillator increases with the ratio. The results also showed that the diffraction influence increases with the length of the amplifier. It was noted that the ultimate output power of the laser system was affected by the ratio of oscillator to amplifier intensity, when the laser was run at the same gain distribution, saturable intensity, amplification of the unstable oscillator, size of the whole laser and various losses of the cavity.