As the key equipment in the new power system, the isolated power supply transformer for DC circuit breaker needs to ensure that its surface electric field strength is within the limits of electromagnetic environment. In order to get accurate calculation results, the differences among transient electric field, electrostatic field and constant electric field are discussed, the influence of air conductivity on constant electric field through the structure of insulating material umbrella skirt is analyzed, and the principle of electric field check for AC/DC hybrid complex equipment is proposed. The electric field simulation analysis and corona check of the power supply transformer are completed. The results show that when calculating the electric field of the power supply transformer, in order to keep a margin, it is suggested to use electrostatic field to check corona inception field of conductor fittings. Constant electric field is used to design the insulation performance of insulating materials. At the same time, the influence of air conductivity should be considered, and the maximum value of the calculation result under air conductivity of 10^-16 S/m(dry) and 10^-12 S/m(wet) should be selected as the reference maximum electric field.