In the design process of switch cabinet, the heat dissipation of current-carrying loop is one of the key problems. By introducing the heat conduction differential equation, Navier-Stokes equations and radiation heat transfer equation, the multi-physical field coupled mathematical models are established for researching the heat dissipation problem in the switch cabinet in this paper. The mathematical models and the finite volume method are presented to compute the three-dimensional thermal and fluid fields of KYN28A-12 switch cabinet. And wall function method is used to deal with convection heat transfer boundary conditions. Then the hexahedral finite element mesh method is used in the numerical calculation. The thermal and fluid fields are calculated respectively in two different conditions, natural convection and forced convection. Compared with natural convection, the temperature in the forced convection is obviously lower. The heat dissipation of the current-carrying loop is increased in the situation of forced convection.