The gain characterization of photonic crystal fiber amplifier is theoretically and experimentally studied by using reverse pumping technique. The gain curves for different pumping powers and different signal lights are obtained experimentally. When the power of the probe light is 6W and the pump power is 160W，the output power up to 104W，has been achieved eventually. It is found in the experiment that when the pump power is increased over a certain value，the output of the amplifier evolves into the unstable state，which in turn degrades the system performance.