【Purposes】 A deep understanding of the pore structure of cement paste is important for improving the performance of cement-based materials. 【Methods】 In this paper, terahertz time-domain spectroscopy (THz-TDS) system was used to test the terahertz time-domain signal of dry cement paste with water cement ratio of 0.3, 0.4, 0.5 and hydration ages of 7, 28 and 56 d in the frequency range of 0.2~2 THz. The refractive index and dielectric constant of cement paste were calculated by using the delay time of time-domain signal. 【Findings】 The test results show that the dielectric constant of cement paste decreases with the increase of water-cement ratio at the same age. The dielectric constant of cement paste with the same water cement ratio does not change obviously with age. The relationship between the effective dielectric constant and pore structure of cement paste is established by using the General Effective Media (GEM) model. Through the fitting of experimental data, the dielectric constant of the solid phase of hydration products in cement paste is 8.73. 【Conclusions】 According to the GEM model parameter analysis, the porosity of cement paste is the most important factor affecting its effective dielectric constant.