Abstract: Currently, in the nuclear safety class DCS cabinet, forced air cooling is the main way to dissipate the heat. However, the life of fans is often shorter, which requires the real-time monitoring of their operation status and regular maintenance and replacement, thus increasing the use cost of the nuclear safety DCS cabinets. Compared with the forced cooling through fans, natural cooling without fans is a more reliable and economical method. This paper introduces a design and analysis method for the natural convection heat dissipation system, and verifies the rationality of the analysis method by Flotherm electronic heat dissipation analysis software. According to this analysis method, a cabinet without fan for nuclear safety DCS FirmSys is designed. The cabinet has been applied in the second phase reconstruction project of Qinshan Nuclear Power Plant.