In order to cope with the severe running conditions of railway vehicles, the cabinet fixed outside the train often needs to be designed as a sealed one, which causes heat dissipation problems. In this paper, the thermal design and heat dissipation analysis of an exterior-mounted electric cabinet were carried out. The heat dissipation scheme using fan and fin radiator was proposed, and the simulation analysis was carried out with ICEPAK. Finally, the heat dissipation design was verified to be effective and reliable by experiments.