Predicting the electrical performance and temperature field of radioisotope thermoelectric generator (RTG) is crucial and essential before they are used in space, a common application scenario. However, building a laboratory to recreate a space environment is expensive and time‐consuming. It is also unrealistic to deploy temperature measurement probes in various components of the RTG. This article aims to establish an approach which combines finite element method (FEM) and experimental measurements in the terrestrial laboratory to solve the problem more effectively: first, using FEM to calculate the temperature distribution of RTG operating in the space; second, realizing the similar temperature distribution of self‐assembly RTG prototype (electrical thermoelectric generator [ETG]) in the terrestrial laboratory by air cooling. The subsequent measurements of electrical performance indicate that the ETG exhibits a maximum power output of 43.41 W and a maximum thermoelectric conversion efficiency of 5.788% in the simulated space environment, aligning well with the values obtained from FEM. This research has the potential to serve as a method for forecasting the performance of RTG in a terrestrial laboratory.