Concrete has complex pore structure characteristics, and a variety of pore structure testing techniques, including nitrogen absorption and desorption method, mercury intrusion porosimetry, gravimetry method, scanning electron microscopy, industrial computerized tomography, etc., were used to characterize the pore structure characteristics of fly ash concrete. Porosity, pore size distribution, specific surface area, and characteristic pore size were studied by MIP and NAD. The tortuosity was calculated by the corrugated pore structure model of the MIP method. The interfacial transition zone of the microstructure is studied by scanning electron microscopy and industrial computerized tomography. The results show that the fly ash content in the range of 10∼30 % could increase micro- and meso-pores in concrete material which is called the refining effect. In the range of 10∼30 % of fly ash content, the three characteristic apertures decreased with the increase of fly ash content. Effect of sample sizes on porosity was gained by NAD and GM methods. The tortuosity of concrete pore structure is between 4 and 9 and fly ash content has no obvious influence on this pore character. SEM imaging of the interfacial transition zone showed obvious defects, and the results of SEM and industrial computerized tomography showed that the width of the interfacial transition zone ranges from 3μm to 168.88μm.