A -16 dB waveguide coupler for the Local Oscillator(LO) signal in 400-500 GHz band is developed in order to meet the application requirements of terahertz sideband-separating receivers, mainly considering the weak coupling of LO signal, the high directivity of Radio Frequency(RF) signal and current milling process accuracy. The characteristic of the coupling degree of the branch coupler is analyzed. The design of a -16 dB waveguide coupler is conducted. The device is processed based on Computer Numerical Control(CNC) technology and the result is discussed. The measured results of both samples show that this coupler can work in 400-500 GHz band (factional bandwidth of 22.5%), obtaining the coupling coefficient for the LO signal in -16~-17 dB, the directivity for RF signal of -1.2 dB, the isolation degree better than -20 dB, and the return loss of each port better than -15 dB. The performance is highly consistent with that of the simulations, which indicates that current CNC technology can meet the high precision requirements for the fabrication of such high frequency waveguide coupler.