In order to meet the increasing cooling demand of high-power power electronic devices, this paper proposes the natural circulation boiling cooling system. This system utilizes phase change process for efficient cooling and uses natural circulation to drive the cooling medium. Based on the theoretical basis of two-phase flow and heat transfer, a homogeneous flow system simulation model is established. The typical representative converter valve of high-power power electronic device is taken as an example to design the natural circulation boiling cooling system of the converter valve. On this basis, the experimental platform of the natural circulation boiling cooling system of the converter valve is built. The simulation and experimental results show that the homogeneous flow model can accurately reflect the two-phase flow process of the system, and the temperature control effect of the natural circulation boiling cooling system is implementable.