A novel mesoporous silica foam (MCF) with a specific surface area of 712.5 m²/g and a pore volume of 2.44 cm³/g is synthesized by using triblock copolymer poly (ethylene oxide, polypropylene oxide and ethylene oxide, P123) as template and TEOS (C₈H₂₀O₄Si) as silicon source. The effect of polyethylenimide (PEI) modified MCF on nanoscale pore structure is studied by positron annihilation lifetime spectroscopy (PALS) and conventional characterization methods, such as N₂ adsorption desorption, transmission electron microscopy, thermogravimetric analysis and Fourier transform infrared spectroscopy. The results show that the synthesized MCF has an obvious disordered mesoporous structure, and a continuous porous network with window connection between the pores is formed. Meanwhile, it can be seen directly that PEI is successfully introduced into MCF pore channels. In order to evaluate the pore size and its distribution more comprehensively, the mechanism of positron annihilation which is highly sensitive to nanometer scale open volumes in PEI loaded MCF is studied. It is found that there are two long life components τ₃ and τ₄, indicating the micropores and mesopores co-existing in the sample. Furthermore, the introduction of PEI molecules results in a significant decrease in τ₃ and τ₄, and the lifetime values are then corrected by using the positron annihilation rate formula in pure gas to calculate the pore size. The results show that the pore size gradually decreases with the filling of the organic molecule PEI. This provides a new insight into the mechanism of regulating the pore structure of MCF by polyethyleneimine modification, as well as the characterization of pore structure in organic-modified mesoporous molecular sieves.