Aiming to provide an accurate method to calculate the refractive index and absorption coefficient of solid powder materials in terahertz region, the effects of porosities of unary and binary powders on the extraction of optical parameters are studied by taking polyethylene(PE) and polytetrafluoroethylene(PTFE) as examples. Based on the effective medium theory, the refractive indexes of PE and PTFE are calculated by using Landau-Lifshitz-Looyenga(LLL) model, and the results are in consistent with the refractive indexes of thermoplastic plates of PE and PTFE. This research is help for constructing terahertz standard spectra database of solid powder as well as the quantitative identification.