Abstract: The development of tank vacuum circuit breaker based on vacuum breaking technology and environmental protection gas insulation technology is an important direction for the development of high-voltage vacuum circuit breaker. In this paper, an electric field model of 126 kV single-break vacuum interrupter and tank circuit breaker is established. The changes of the electric field inside the vacuum interrupter in the tank were explored from three aspects: electric field distribution on the con-tact, effective area of the contact, and symmetry of electric field distribution. The results show that the electric field distribution on the moving and fixed sides of the vacuum interrupter is asymmetrical in the tank, and the electric field strength on the high potential side increases and the electric field strength on the low potential side decreases. In view of this situation, an equivalent circuit model is established to analyze from the circuit level. The results show that the increase of stray capacitance to ground causes the potential of the main shield to decrease. Furthermore, by setting the potential of the main shield to a series of fixed values, the influence of the potential of the main shield on the electric field distribution inside the vacuum interrupter was studied. When the potential of the main shield is the same, the internal electric field distribution of the vacuum interrupter in the metal tank and the single vacuum interrupter is basically the same, which provides a reference for the optimization of the internal electric field of the vacuum interrupter in the tank circuit breaker.