Plasma filling can significantly improve the efficiency and power of a vacuum device. In this paper, we first analyze the dispersion properties of a plasma-filled metal-photonic-crystal slow-wave structure (SWS), and then investigate the interaction procedure between a relativistic electron beam and the Cherenkov radiation in the plasma-filled metallic-photonic-crystal by the particle in cell method. We pay our attention to the influences of plasma density, cathode voltage, and guiding magnetic field on output frequency and power. The results show that the electric field strength in the SWS increases obviously at a fixed plasma density of 50 mTorr (1m mTorr=0.133 Pa). The device works at a stable single TM01 mode due to the good mode properties of the metal photonic crystal even if plasma is filled in it. The maximum value of Ez field along the z axis of the device increases from 46.34 MV/m without plasma to 79 MV/m with plasma. The value along the x axis increases from 136 MV/m without plasma to 185 MV/m with plasma. The working frequency (35.5 GHz) of the device, obtained from simulation, is consistent with the theoretical estimation (35.4 GHz). The power increases with the cathode voltage between 500 kV and 600 kV while the frequency increases only a little. When the magnetic field B increases, the output power first increases and then decreases. But the frequency is not affected due to the dispersion property. The output power of the device increases 20% when the air pressure increases from 0 to 100 mTorr. However, there is a pretty distribution of the field Ez along the angular direction only in an appropriate plasma density around 50 mTorr. According to the theory and simulation, the output power and efficiency can be improved in an appropriate range of plasma density. These results provide a basis for developing the plasma-filled vacuum devices.