Microstructural characteristics are the essential reflection of macro phenomenon. Microstructure of EthylenePropyleneDiene Monomer (EPDM) waterproof sheet applied to underground engineering is studied by using Xray CT technology; and the structural properties of pores in the material are discussed. The NaCl solution is prepared to simulate the groundwater environment. The EPDM waterproof sheet in groundwater environment is scanned with microCT; the acquired five images are processed with Matlab software; the concept of pore equivalent diameter is introduced and the relevant pore parameters are analyzed. The results show that: 1) There is only little difference among pore parameters of the five images. 2)The pore diameters are mainly in the range of 0-60 μm; and the diameter of 20 μm is the turning point of pore number distribution. 3) The large and small pores have a significant effect on pore area distribution. It is proved that the microCT is a feasible method for studying microscopic pore characteristics of waterproof material in underground engineering, so as to provide a basis for the farther research on damage mechanism of waterproof material.