The circuit breaker equipped with closing resistance is an effective measure to reduce the switching overvoltage. This paper introduces the structural characteristics of 800 kV tank circuit breaker.The closing resistance switch is connected in parallel with the closing resistance

which is connected in series with two main arc extinguishing units. The movement principle of closing resistance switch is designed with variable length arm

which achieves the movement characteristic of slow first and then fast during closing operation process

and ensures 8～12 ms insert time of closing resistance. The moving contact of the closing resistance switch is designed with spring oscillator

which realizes it opens later than the main arc extinguishing units

and ensures the arc extinguishing function is completed by the main arc extinguishing unit. The rated thermal capacity of closing resistance of 800 kV circuit breaker is 39.7 MJ. For operation sequence C（1.3 E）—3 min-C（1.3 E）—30 min-C（1.3 E）—3 min-C（1.3 E）

its safety factor is1.51 times;for operation sequence C（2 E）—30 min-C（2 E）

its safety factor is 1.27 times. The electric field distribution under out-of-phase and LIWV is simulated

and results show that its insulation performance meets the requirements. The standards for closing resistance of circuit breaker should be improved.