Considering the overmoded structures of high-power Terahertz(THz) sources are often electrically large, it’s difficult to compute the radiation of THz antennas on a personal computer due to over long time and prohibitive computation resources. A parallelized finite-difference time domain (FDTD) algorithm based on MPI platform and virtual topology structure, combined with theory of guided waves, is presented for analysis of the radiation of the large THz conical horn excited by mixed-mode souce. Cartesian virtual topology structure is firstly defined by MPI_CART_CREATE( ) function based on MPI platform. And MPI_CART_SHIFT() function is used to define the position relations of the subdomains. Then FDTD method is used in each subdomain. The absorbing boundary of the whole FDTD domain is uniaxial perfectly matchedlayer (UPML), and that of the waveguide is convolutional PML(CPML). Synchronous communication mode is used in parallelized FDTD between the adjacent subdomains. The coefficient of field components for each mode source can be got based on the given power of each mode. Thus the mixed-mode excitation source can be set by the coefficient and each mode’s initial phase. Examples of an electrically large THz horn with 4 or 6 modes mixed excited are given in this paper. Considering the universal characteristic of FDTD method, the method shown in this paper can be used to simulate the radiation of other kinds of THz antennas with mixed-mode exicitation source. And it’s useful for the design of those structures.