W-band target simulation system is usually suffered from out-of-band noise. In this paper, a w-band microstrip band-stop filter based on lowpass prototype and quarter-wavelength microtrip is proposed. The equivalent circuit analysis method and the 3D electromagnetic field simulation are used to design and optimize the filter. The layout of designed filter is millimeter size in overall. Simulation results show that the filter is of low inband insertion loss，high out-of-band noise suppression and a steep transition band. Therefore, the band-stop filter can be used on W-band target simulation system for out-of-band noise suppression.