A gain-flattened Raman fiber amplifier was proposed by using the method of cascading photonic crystal fiber, which improves the Raman gain coefficient and reduces the gain flatness of the traditional Raman fiber amplifier. Based on steady-state analysis theory of the stimulated Raman scattering effect, the photonic crystal fiber Raman gain spectrum and established the theoretical model of this Raman amplifier were analyzed. Through solving the coupled equation, the constraint condition for realizing gain-flattened is deduced, showing that the fiber length and pump power are two parameters affecting gain flatness of Raman fiber amplifier. Numerical simulation results show that a photonic crystal Raman fiber amplifier with 21 dB high gain and only 0.14 dB gain flatness over the bandwidth range of 1 508 nm to 1 544 nm, can be obtained. It will play an important role in the design of high efficient optical fiber communication systems.