The multilayer mirrors are primary reflectors used in the normal incidence telescope for 30.4nm radiation

but they also have very high reflectivity in ultraviolet

visible light and near infrared wave band

therefore it is very necessary to use some special filter to remove these longer radiations. It is important for the performance of filter to select proper material and thickness. Confirm that aluminum is the best material for 30.4nm filter by computing the linear absorption coefficient for several kinds of materials as a function of wavelength based on atomic scattering factor. And considering the transmission and hardness

the filter’s thickness is decided. Improved the spectrum measurement instrument made of Mcpherson model 247 grazing incidence monochromator and gas hollow cathode source

it is fit for transmission measurement and the wavelength scanning error is ±0.17~±0.97 .Measured the prepared filter’s transmission range from 25.6nm to 1000 nm using it，the result shows its transmission is 58.85% at 30.4 nm. It is demonstrated that the filter can be used in the 30.4nm imaging system since the further measurement dedicates it can prevent UV、visible and near infrared light effectively.