Owing to its advantages of high specific surface area, large pore volume, adjustable pore size, good thermal stability and relatively low cost, SBA-15 has a wide range of application prospects in adsorption, separation, catalysis, nanomaterials and other fields. And the use of organic functional groups to modify SBA-15 has become one of the hot spots of research on materials, but the introduction of organic functional groups will inevitably affect the pore structure of material, affecting its performance. Therefore, how to more comprehensively characterize the pore structure of material has received much attention. In this work, small angle X-ray scattering (SAXS) technique is used to characterize the pore structure of PEI/SBA-15 mesoporous molecular sieve. The pore structure and periodicity information of PEI/SBA-15 are obtained by using correlation function and string length distribution theory, and compared with those obtained by positron annihilation lifetime spectroscopy (PALS) technique. The results show that the periodic structure of PEI/SBA-15 mesoporous molecular sieve does not change significantly with the increase of PEI mass percent, and the pore size of PEI/SBA-15 mesoporous molecular sieve only decreases from 8.3 nm to 7.6 nm by the chord length distribution function. Two long-life components , τ ₃and τ ₄, are obtained by PALS, and τ ₃reflects the random pores structure in SBA-15 matrix, while τ ₄denotes the size of SBA-15 hexagonal pores. Compared with the results of SAXS, the mesoporous pore size obtained by PALS technique shows the same trend. By combining SAXS technique and PALS technique, the evolution of material microstructure can be revealed in more depth, thus providing a unique method for studying the structural characterization of functional nanocomposites in the future.