Aiming at the problem that the oil well cement sheath is easily corroded by CO2 in the high temperature environment of the formation, a water-based epoxy resin WEP was prepared using sodium styrene sulfonate and epoxy resin E54 as raw materials. FTIR results showed that the modified product was the target product. The centrifugal test showed that the WEP emulsion could be stabilized at 6000 rpm for 30 min, and the microscope was magnified 150 times to observe that the WEP emulsion was uniformly dispersed and the particles were small, indicating that the WEP had good self-emulsifying properties. Thermogravimetric analysis showed that the decomposition temperature of WEP was 295 ℃, and the thermal stability was good. The high temperature corrosion test results show that the compressive strength decline rate of the contrast cement stone after 90 d corrosion is 62.0%, and the compressive strength decline rate of the WEP modified cement stone is only 14.7%. The thermogravimetric analysis of the cement stone shows that the CaCO3 content in the contrast cement stone is about 4.02%, while the CaCO3 content in the WEP-modified cement stone is only 0.25%. The XRD spectrum of the cement stone after corrosion shows that the characteristic peaks of some hydration products in the contrast cement stone have disappeared, and a large number of characteristic peaks of CaCO3 appear. The main components of the WEP-modified cement stone are still C-S-H and other cement hydration products. The SEM results show that WEP forms a polymer film in the cement stone, wraps the hydration product, reduces the contact between the hydration product and CO2, and improves the CO2 corrosion resistance of the cement stone.