The modified polypropylene(PP) material has been widely used in dashboard, bumper and other auto parts because of its characteristics of flame retardant, high impact resistance, etc. It is a necessary means to improve the quality of auto parts by using the nondestructive testing technology to detect the modified PP material. A transmission terahertz time-domain spectroscopy(THz-TDS) imaging system and a reflective THz-TDS imaging system were built. The optical parameters of modified PP materials were extracted by adopting the transmission THz-TDS system. The refractive index, one of the optical parameters in terahertz band can be solved through the mathematics model, its value was 1.53. A kind of modified PP material sample with flat bottom holes was designed. The reflective THz-TDS imaging system was adopted for measuring the sample, the deconvolution method was used to filter the time domain THz signal, which largely improved the signal-to-noise ratio. What′s more, THz-TDS tomography imaging technology was proposed based on the time of flight. Combined with the measured refractive index, the 3D structure of the modified PP material was reconstructed with the time of flight tomography imaging technique, the thickness accuracy is 0.01 mm.