A cascaded medium thin film filter used for flattening the gain of erbium2ytterbium co2doped waveguide amplifier is presented. It consists of some different filtering unit cell with different central wavelength and band2 width ,and each of which can suppress a certain gain peak at a specific wavelength.An example,‘= ’type wideband erbium2ytterbium co2doped phosphate glass waveguide amplifier integrated with cascaded medium thin film filter ,is given , and the transmission function of that is obtained. It demonstrates that this cascaded medium thin film filter has good filtering performance.