Abstract: The thermal analysis is an important part in the spacecraft thermal design and for the in-flight temperature prediction. The usual way is through solving the energy balance equations, also called the thermal network equations, of an orbiting spacecraft. And the iterative method is usually used in view of the typical nonlinear nature of the equations. However, at present, there is no theoretical criterion to judge the convergence of the iteration process. Based on the engineering practice of the spacecraft thermal design, an approximate linearization method is proposed in this paper. Then, the convergence condition is analyzed, to obtain the mutually-constrained relationship between the time step and the thermal parameter of the subject node and the time step for the calculation. Furthermore, the influence of the time step on the calculation efficiency is studied, and the optimized time step is obtained. Lastly, calculations are carried out on an established typical spacecraft thermophysical model, to verify the proposed method and the related research conclusions.