Raman fiber amplifier (RFA) can effectively improve the sensing distance of phase sensitive optical time domain refl-ectometer(Φ-OTDR) system. In order to further improve the system sensing distance of Φ-OTDR, the particle swarm optimiza-tion(PSO) algorithm is used to optimize the pump optical power value of the third-order bidirectional RFA. The theoretical farth-est sensing distance of Φ-OTDR system based on third-order bidirectional RFA is calculated and a simulation experiment is designed. The simulation and analysis results show that when the detection pulse width is greater than 200 ns, the Φ-OTDR system can reach more than 200 km through the third-order bidirectional RFA amplification sensing distance.