Based on the classical half-wavelength filter theory, a half-wavelength magnetically coupled rectangular waveguide bandpass filter is studied and designed for terahertz communication system. The simulation results show that the center frequency of the filter is 118 GHz; the passband is 114.3~123 GHz; the relative band-width is 7.4%; the attenuation at 131.8 GHz is more than 30 dB; the passband insertion loss is less than 0.8 dB; and the pass-band return loss is more than 20 dB. After the physical test, the test results are basically consistent with the simulation results. This filter bears a simple structure and a low fabrication difficulty.