UMo-Zr monolithic nuclear fuel elements, due to their high thermal conductivity, large heat transfer area, and strong ability to contain fission products, are one of the main fuel elements in nuclear research test reactors worldwide. However, UMo-Zr monolithic nuclear fuel elements may exhibit blistering phenomena under high-temperature, high-pressure, and strong irradiation environments when they reach a certain burn-up, which affects the performance and safety of the fuel elements. The blistering of fuel elements involves a very complex mechanism. This study aims to overcome the real-time coupling technology of internal pressure, cavity volume, and temperature during the blistering process, thereby systematically and completely simulating the evolution mechanism of blister size in UMo-Zr monolithic nuclear fuel elements after blistering. In the study, the coupling mechanism in the blistering of UMo-Zr monolithic nuclear fuel elements was first investigated, a mathematical coupling model for the evolution of blister size was established, and corresponding numerical simulation methods were developed. Based on this numerical simulation method, the key factors affecting the evolution of blister size during the blistering process were systematically studied, and the behavior of blister size evolution was obtained. The results show that the numerical simulation method established in this study can effectively simulate the evolution process of blister size, revealing the coupling relationship between internal pressure, cavity volume, and temperature. This study not only enriches the theoretical basis for the evolution of blisters after blistering of UMo-Zr monolithic nuclear fuel elements but also provides new perspectives and methods for research in the field of nuclear fuel safety, playing an important role in the safe utilization and sustainable development of nuclear energy.