Pore solution expression is the most commonly used method to obtain aqueous phase in cementitious material. However, the high pressure applied on the sample may affect solid phase composition. This paper presents an experimental study on the chemical and mineral composition of cement paste before and after the expression. Results indicate that a small part of the alkali contained in samples can be excluded during the pore solution expression, mainly depending on alkali concentration in the pore solution. Due to the expulsion of interlayer water in C-S-H under high pressure, the pore solution expression reduces bound water content measured by TGA. The portlandite content determined by TGA is not affected by pore solution expression, but it leads to an overestimation of portlandite in QXRD, because of the enhanced preferred orientation of the (001) plane under the applied high pressure. In addition, the size of portlandite crystal decreases to some extent due to the creep caused by pressure. The content of hemicarbonate phase is found to decrease slightly after pore solution expression, which may be aroused by the increased solubility under high pressure.