Miniature circuit breaker (MCB) is a core device that implements overload and short circuit protection functions in terminal distribution networks. Its overload protection function relies on the bimetallic strip being bent by heat to drive the contact to trip. Therefore, studying the influencing factors of bimetallic strip heating is of great significance for a deep understanding of the MCB overload protection process. This article establishes a three-dimensional finite element model of MCB thermo-electric coupling, investigates the steady-state temperature field distribution characteristics of MCB, and studies its effect on the temperature of bimetallic strips by changing the ambient temperature. The results show that the temperature of the bimetallic strip will increase as the ambient temperature rises. The environmental temperature is related to the surface heat dissipation coefficient. To simulate the effect of environmental temperature on bimetallic strips and MCBs through simulation methods, further research should be conducted on the relationship between temperature and surface heat dissipation coefficient. Other heating and cooling issues should also be studied to fundamentally explain the impact of environmental temperature on the heating and cooling process.