In the process of cementing mud slurry preparation， ultra-high mixing speed tends 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 micro-computed tomography test and scanning electron microscope test， the changing law of microstructure of cement slurry can be grasped and the mechanism of defoamer’s action on cement slurry can be clarified. The research results show that the compressive strength of cement stone shows a trend of increasing first and then decreasing， while the porosity shows a trend of decreasing first and then increasing. The cement stone 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 the Schiller’s Function （R²=0.98）. The incorporation of defoamer does not change the mineral composition and hydration products of oil well cement， and it mainly reduces the surface tension of the bubbles to 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 the “defoamer holes”， which will lead to the increases of the cement stone porosity and the decreases of the compressive strength.