In the past two decades, with the rapid development of the terahertz science and technology, the interaction between terahertz pulse and material has attracted much more scientific interest. In this paper, an accurate optical parameters extracting method was theoretically derived and experimentally demonstrated, a HDPE sample was measured by two similarly experimental scheme with terahertz time-domain spectroscopy. To compare the results of the regular method and the different optical path (DOP) method, the refractive index and the absorption coefficient from 0.5-2.5 THz were obtained, respectively, and the results show that the refractive index and the absorption coefficient of measured sample are quite different. The refractive indexes obtained by regular method and DOP method are 1.505 and 1.483 respectively. The absorption coefficient measured by the regular method has a peak in 1.465 THz, while DOP method′s absorption coefficient has two peaks both in 1.166 THz and 2.431 THz. The different optical path method can reveal the absorption peak of the materials which can′t be revealed by the regular method, and provide a more precious measurement of material optical parameters.