Abstract: For 126 kV gas-insulated metal-enclosed switchgear, the arc-extinguishing chamber of the three-phase circuit breaker is in the same shell, and the three-phase synthesis test must be carried out when the breaking test is carried out for the products of this common box structure. However, the relevant standards only specify the transient recovery voltage parameters of the first open pole. Based on the 126 kV three-phase composite test, the transient recovery voltage parameters of the first open pole are analyzed and calculated under the condition of neutral point non-effective grounding (the first open pole factor K_pp=1.5) and the two-pole transient recovery voltage parameters under the condition of neutral point effective grounding (K_pp=1.3) transient recovery voltage parameters of the second breaking pole and the third breaking pole. And according to the equivalence conditions and requirements of the synthetic test loop and the direct test loop, the parameters of the test loops under the conditions of K_pp=1.5 and K_pp=1.3 are calculated for the transient recovery voltage of different breaking poles. PSCAD software is used to analyze the calculated parameters of each test loop. Simulations are carried out, and the simulation results show that the transient recovery voltage generated by the calculated test loop meets the expected parameters. In the large-capacity laboratory, the three-phase composite short-circuit breaking test verification under the condition of 126 kV/40 kA neutral point non-effective grounding is carried out, and the test results verified the correctness of the voltage loop parameter calculation.