Introduction  In order to analyze the pressure relief process and diffusion pattern of SF₆ leakage gas in a 6 km span in the UHV GIL， a type of 1D and 3D hybrid CFD numerical method is proposed. 1D Flowmaster was used to study the influence of different pore size of leakage port on SF6 pressure relief process. 3D Fluent was used to analyze the concentration distribution and diffusion pattern of SF₆ in the pipe gallery.

  Method  In order to verify the accuracy of the hybrid numerical method， the calculation results of 1D Flowmaster and 3D fluent for SF6 pressure relief process of the same pipe gallery structure was compared.

  Result  The compared simulation result showed that Flowmaster simulation of pressure relief process has the characteristic of fast calculation speed and high accuracy.

  Conclusion  Combining with the requirement of ventilation design， the classification of light， medium and heavy leakage accidents based on the interval of leakage aperture， the corresponding diffusion form， concentration distribution and other important data are obtained， which can provide guidance for practical application design and accident handling.