A new terahertz (THz) temperature sensor, which is based on surface plasmon resonance (SPR) by prism coupling excitation at THz frequency, is proposed. A novel prism coupling structure is designed for the temperature sensor by using unique features of thermo-optic effect for silicon at 1 THz and the high THz emission effiency of InSb. The medium thickness for the THz temperature sensor structure is analysed to get the optimal value by the finite element method, so that the sensitivity can reach optimal value. The optimal thickness of each layer is given, and the sensitivity of THz temperature sensor can reach $9.75 \times 10^{4^\circ}/^{\circ}$C , and FWHM of the SPR spectrum can reach 0.073$^\circ$, within the detection range of temperature, 0−80$^\circ$C, and the relationship between SPR angle and temperature is obtained within the detection range of temperature.