Based on design principle of thin-film optics, to the special needs for output spectrum shape and gain characteristics in the process of femtosecond laser pulse amplification, a type of femtosecond laser spectrum filter is designed through optimum calculation to thin-film system. The filter can control intensity of laser spectrum and distribution of laser frequencies for a special purpose, and acquire the special femtosecond laser spectrum shape which is needed. Our results show that the femtosecond laser spectrum filter can be used in the femtosecond laser chirp pulse amplification system.