In the process of cementing mud slurry preparation, ultra-high mixing speeds tend to introduce air bubbles, resulting in difficulties in formulating the cement slurry to the correct density, which in turn can adversely affect the slurry properties and the sealing integrity of the cement ring. In this paper, we take the lipid-based defoamer X60L as an example to investigate the effects of different dosages of X60L defoamer (0%, 0.5%, 1%, and 1.5%) on the compressive strength of oil well cement. Combined with thermogravimetric analysis, X-ray diffraction analysis, X-ray computed tomography test and scanning electron microscope test, we can grasp the changing law of microstructure of cement paste and clarify the mechanism of defoamer"s action on cement paste. The research results show that the compressive strength of cementite shows a trend of increasing first and then decreasing, while the porosity shows a trend of decreasing first and then increasing. Cementite has the lowest porosity and the highest compressive strength when the defoamer dosage is 0.5%. The larger the porosity of cement stone, the smaller the compressive strength, and the compressive strength and porosity were obviously negatively correlated and conformed to Schiller"s function (R²=0.98). The incorporation of defoamer does not change the mineral composition and hydration products of oil well cement, and the defoamer mainly reduces the surface tension of the bubbles, which makes it difficult to maintain the bubbles and thus break them. However, with the increase of defoamer dosage, too much defoamer will hinder the contact between water and cement, inhibit the hydration of cement, and introduce ‘defoamer holes’, so that the porosity of cement stone increases and the compressive strength decreases.