Abstract: Combined voltage test is one of the most important items of dielectric type test for circuit breaker. Due to its higher insulation level of circuit breaker at 1100kV rated voltage and bigger grading capacitors, its dielectric type test is a challenge for laboratories. Applying EMTP software, combined voltage tests under different cases were computed and test plans were determined. Results show: for the cases of SIWV+AC, the dropped voltage can be compensated only by increasing the voltage of AC side; for the cases of LIWV+AC, the dropped voltage can be partly compensated by increasing the voltage of AC side, partly compensated by increasing the voltage of LIWV side. Based on the computation, according Chinese specification GB/Z 24838-2009, High Voltage Apparatus Quality Test & Research Center of Machinery Industry fulfilled the combined voltage dielectric type test of 1 100kV SF₆ circuit breaker developed by New Northeaset Electric(shenyang) HV Switchgear Co.Ltd successfully.