Abstract: Based on the theory of mode coupling, the paper deduced the timedependent couplingwave equationd that the modes satisfy in tapering circular waveguide, and presented the expressions of coupling coefficients of the equations. The metallic wall loss was considered in the process. According to the equations, we write a code for simulation of the multimode transmission property in tapering circular waveguide. Firstly, using the code, mode loss in circular waveguide in THz region was computed.It is found that when studying the multimode transmission property in tapering circular waveguide,the effect of metallic wall impedance loss must be considered.Secondly,we analyzed mode coupling numerically. This attests to the existence of the coupling mechanism due to fininte conductivity of the meta