This paper demonstrates a novel tunable CW THz source design, which can generate a THz wave with frequency from 0.75 THz to 1.75 THz, based on nonlinear laser frequency mixing on a photoconductivity structure. The device consists of a semi-insulating GaAs substrate with silicon nitride antireflective coating, an epitaxially low-temperature-grown In0.52Ga0.48As thin film and an Archimedean spiral metal antenna with two electric contact pads. The Multiphysics software COMSOL was used to simulate the THz source system. Different dimensions of the metal antenna have been modeled. It is shown that when the dimensions of the metal antenna are given as inner radius 15 μm, outer radius 95 μm, spacing between each turn 10 μm, width of each arm 10 μm, and turns number 2, respectively, the spiral antenna’s frequency band is from 0.5 THz to 3.2 THz, which can sufficiently cover the frequency range of a desirable wideband tunable THz source. This paper also presents some related THz radiation simulation results based on the proposed structures.