For steady-state temperature field simulation during cooling stage of plastic injection molding, when the traditional finite volume method (FVM) is used to solve the equivalent heat flux on the cavity surface, the approximate node temperature gradient is often based on the average value in the control volume, but it may not be equal to the local node temperature gradient on the boundary surface. In addition, the calculation of heat flux on each surface in the control volume is independent, which may lead to the non-conservation of heat energy on the control volume. To this end, the improved FVM is modified by taking the heat conservation of steady-state temperature field as a prerequisite and calculating the heat flow of cavity surface based on the node control volume. A numerical simulation software of C++ language based on FVM is developed, which compares the steady-state temperature field of plastic injection molding simulated by the traditional FVM algorithm and the improved FVM algorithm with the results of commercial softwares. The results show that the traditional FVM algorithm can simulate the rough temperature distribution of injection mold, but there is a large calculation error. By improving the equivalent steady-state heat flux calculation method for cavity surface, the calculation error is greatly reduced, which proves that the improved FVM algorithm can predict more accurately the steady-state temperature field in cooling stage.