Abstract: Aiming at the problem of plasma stealth for aircraft inlet, we create a three-dimensional model for cylindrical inlet. A finite element model is used to solve the wave equation to gain the RCS (radar cross section) of inlet whose inner wall is covered with uniform plasmas. The model operates at a collision frequency of 1-100 GHz, an electron number density of 1016-1018 m-3, a microwave frequency of 1-3 GHz, and an incident angle of 0-80. The results show that the attenuation is significant when the inlet is covered with plasmas and decreases with the microwave frequency; there are several attenuation peaks because of the cavity geometry structure; the attenuation increases with the electron density, but it can deteriorate while the electron density is too high; the optimum collision frequency is related with the microwave frequency and the electron density, but it is generally about 9 GHz; the attenuation increases with the thickness of plasma, but it can deteriorate because of the reflection on the interface between the plasma and the air while the electron density is too high; the attenuation can be significant in whole microwave band (1-3 GHz) at the appropriate incident angle and plasma density, which is important for designing plasma source to applied the plasma stealth.