Gain flatness of erbium doped fiber amplifier (EDFA) is a critical issue for wavelength division multiplex (WDM) system, and realizing this function by photonic crystal is a new scheme proposed in this paper. We designed the gain flatness filter satisfied with special condition by analyzing the principle of gain flatness of EDFA and the different reflection character of one dimension (1-D) photonic crystal whose reflectivity is different at different wavelength. Taking the typical gain spectra of EDFA for example, we optimized the period of optical crystal conveniently and exactly using the genetic algorithm and finally designed the gain flatness filter matched with the gain spectra of EDFA. The results show that the designed cascaded 1-D optical crystal filter can flat the gain of EDFA in the range of 1530～1560 nm and the non-flatness degree is about ±0.6 dB.