Thermal stability is a key indicator for polymeric materials. Using Terahertz Time-Domain Spectroscopy (THz-TDS) technology combined with a temperature control device, two types of thermoplastic polyurethane elastomers (TPU) solid samples are tested. At room temperature, there are differences in the terahertz absorption coefficients and refractive index values of different types of TPU. During the process of heating from 20 ℃ to 160 ℃, as the temperature increases, the terahertz absorption coefficient gradually increases, while the refractive index decreases. The turning point of the linear fit corresponds to the reported Vicat transition temperature of the material, and the sample with higher thermal stability maintains its optical constants more stable after heating and cooling. The research results indicate that terahertz spectroscopy technology can provide a new approach for detecting the thermal stability of polymeric materials.