Compared with L band, the gain flattening of erbium doped fiber amplifier (EDFA) in C band should be conducted by optical filter. Due to unmatched bandwidth, the conventional long-period fiber grating (LPFG) cannot efficiently reduce the fluctuation of gain spectrum. Aiming to improve gain flatness of EDFA, a novel scheme named composite fiber grating is proposed which combining LPFG and fiber Bragg grating-pair (FBG-P) with weak reflectivity. Moreover, the key parameters of CFG are optimized by simulation, and its performance of gain flattening is also experimentally tested. The numerical results show that, the proposed scheme achieves controlling the fluctuation within ±0.45 dB in the range from 1520 nm to 1560 nm, and the improvement on gain flattening reaches 49.31% comparing with the scheme based single LPFG.