A multiple fields’ coupled model of new magnetic controlled DC plasma torch, which was used for CVD diamond film, was presented. In this model, the effects of electric field and magnetic field on the flow field and temperature field were taken into account, and the fluid dynamics equations were modified by the addition of some source terms relating to electromagnetic field, such as Lorentz force, joule heating, and radiative cooling. Conversely, the generalized ohm’s law was used to solve the current density, which reflected the effects of flow field and temperature field on the electric field and magnetic field. In addition, the rest Maxwell’s equations and external solenoid magnetic field equation were also modeled. In order to know the effect of external magnetic field on the torch, the current intensity of external solenoid was chosen to simulate its influence on the flow and heat transfer in the torch. Results show that external magnetic field plays a part in stirring the plasma, which is advantageous for the preparation of diamond film. The larger the external solenoid current intensity is, the better the uniformity of the temperature and velocity of plasma is.