The insulation level of high-voltage switchgear will affect the safe operation and breakingcapacity of the equipment. Therefore, it is necessary to analyze the electric field distribution whendeveloping high-voltage vacuum circuit breakers ( VCB). Based on the prototype structure of363 kV fast VCB, a three-dimensional finite element electric field calculation model was estab-lished. The electric field distribution of closed and open state with rated voltage were calculatedrespectively. The electric field of the arc extinguishing chamber, insulator and main fittings were ana-lyzed. The results show that the maximum electric field of grading rings, and post insulator are1.92 MV/m and 0.36 MV/m respectively. The maximum electric field of break is 3.5 MV/m at the open gaps of 20 mm. Electric field of other parts are within the limitation. The overall structure of the circuit breaker is designed reasonably to meet the insulation requirements.