Abstract: To overcome the shortcomings of polarizers currently used in terahertz frequency region, which are expensive to produce, difficult to be processed, and easy to generate Fabry-Perot echo effect, a new terahertz polarizer was presented by means of Brewster effect of a stack of silicon wafers. According to the principle that the different oil with different compounds and different concentration has the different optical activity of terahertz wave, by using the polarizer, the optical activity of 97^# gasoline, diesel oil and the mixed oil samples were measured to be identificated. The results show that, the extinction ratio of the designed polarizer is higher than 1059 and transmittance is higher than 99% and the polarizer can be used in 0THz~3THz frequency region. By using the silicon wafer polarizer, the optical activity of 97^# gasoline, diesel oil and their mixture is measured. And the qualitative and quantitative detection of oil is realized. The design of polarizer meets the requirement of polarizer in terahertz frequency region.