During multi-stage fracturing, the wellbore casing is subjected to multiple rounds of pressure shocks and complex mechanical environments due to thermal stress cyclic loads, making it crucial to clarify the forces on the casing during multi-stage fracturing for predicting its remaining life. Based on wellbore flow theory and heat transfer principles, considering the number of pressure cycles and the alternating wellbore temperature changing rules during multi-stage fracturing, a three-dimensional finite element model of the casing-cement sheath-formation combination under multi-stage fracturing conditions was established. It analyzed the force distribution pattern on the casing during multi-stage fracturing and predicted the stress changes in the casing under different fracturing stages. Results show that the wellbore temperature decreases with fracturing time and rapidly increases in a short period after stopping fracturing, later slowly returning to near the formation temperature. Considering temperature effects, the remaining strength of the casing in multi-stage fracturing shows a cumulative increase trend with the number of fracturing stages, with the casing Mises stress increasing by 24.72% after 13 stages of fracturing compared to the initial stress. The temperature effect during CO2 multi-stage fracturing is one of the key factors affecting casing stress that cannot be ignored.