In order to analyze power conversion efficiency of forward pumped fibre Raman amplifier(FRA), by the solving differential coupling equations, the theoretical expression of power conversion efficiency was presented. The effect of different parameters on forward pumped FRA’s power conversion efficiency was detailedly investigated. It was found that power conversion efficiency will increase when the fibre length or Raman gain coefficient per unit area increase, until it has reached the maximum. Initial signal power has a little influence on power conversion efficiency when it is small, and power conversion efficiency increases quickly when initial signal power is larger. The relation between power conversion efficiency and initial pump power is para-curve. Power conversion efficiency decreases when the frequency rate of pump and signal is larger.