The power loss model optimization of open frame DC/DC power supply module is introduced based on active-clamp forward topology. The calculation of core loss and line loss is shown and the temperature factor according to the load change is introduced, which make the loss estimation more precise in the whole load range. Considering the full load efficiency, no load loss and cooling requirements, the systematic optimization ideas and quantitative analysis of specific methods are also given. At the same time, thermal analysis and optimization design are given along with the temperature estimation based on the power loss model. The 18—36 V input and 5 V/150 W output DC/DC module power supply is designed and some experiments are performed. The experimental results verify the correctness of the theoretical analysis.