System simulation is important to the optimal design of terahertz radar experiment systems. In order to validate a multiband terahertz radar system，this paper presents a conceptual design and analysis of a 335 GHz terahertz radar system. Then the modules of mixing, frequency multiplication, and quadrature demodulation are designed based on Matlab/Simulink, which form a terahertz radar system. Thereafter a simulation is performed with three point targets as examples, and the spectra of radar signals in each stage are obtained as well as the range profile of the point targets. The range profile and resolution are consistent with the theoretical values. A brief analysis on error and problems occurring in the simulation is given.