In order to study the change of average transmitting power of pulse signal in different order of pulse position modulation(PPM) at the transmitter end in optical communications, this paper uses a two-stage amplification system based on the theoretical analysis model of Yb-doped fiber amplifiers. By optimizing the length of gain fiber, the average output power characteristics of optical emission system with different modulation orders are simulated. The simulation results show that under the limitation of pump power, the optimal gain fiber length under different modulation orders will be affected by the pump power. As the modulation order increases, the optimal gain fiber length also increases, but it has little effect on the average output power; when the hardware conditions of the fixed optical transmission system are unchanged, the average output power of the signals at different modulation orders is not equal, the average transmission power of 128PPM is only about 51% of 4PPM, and the transmission power loss is nearly 3 dB.