Gainflatness is a crucial parameter in wavelength division multiplexing system design. In this paper, the gain flattened theory by using multistage cascaded fiber was presented. The effective distance and Raman gain coefficient constraints were calculated. When spectrum was fitted by linear lines, the gainflattened condition was simplified. The optimal gain value was obtained by cascading two stage fibers. The pure silica fiber and phosphorusdoped fiber were cascaded as the typical example. The results show that with the increasing length of the first stage fiber, the signal gain was firstly increased and then reduced; when the first fiber length was 7.15 km, 5.78 dB average maximum gain and 0.45 dB gain flatness were provided by Matlab. Pump power and signal gain was positively correlated. Three stage fibers were cascaded with different materials as the last example. 7.74 dB average gain and 0.57 dB flatness were verified by Matlab which proves the accuracy of the theory. This scheme provides a novel way to design gain flattened Raman fiber amplifier.