On-off gain characteristics of bi-directionally pumped Raman fiber amplifier are numerically simulated through shooting method. The results show that the on-off gain is approximatively independent of forward pump power percentage under constant total pump power. The results are coincided with the analytical solution under small signal power. When the signal power increases till the pump power depletion causing by signal can not be ignored, its on-off gain slightly increases with enhancement of forward pump power percentage under constant total pump power. In addition, the distributions of the signal power, forward pump power, backward pump power and total power along fiber under various input signal power and various forward/back pump power are given through diagram.