In order to analyze the threshold characteristics of backward-pumped fibre Raman amplifier(FRA), signal power distribution along a fibre with shooting method and Runge-Kutta method of the coupling equations of backward-pumped FRA was provided. Then, the theoretical expression of backward-pumped FRA’s was obtained with considering the practical factors, and the effects of every parameter on threshold of backward-pumped FRA were investigated. It is indicate that, the threshold will decrease when the initial signal power or the effective fibre length or Raman gain coefficient increase, and the threshold will increase when the efficient area of fibre increases.