Abstract: The issue stemming from the short circuit in the tunnel cable system has garnered significant attention. The development of a fault model for tunnel cables and the optimal design of the working shaft are crucial factors for ensuring the safe and stable operation of the system. The tunnel cable system is divided into a π-type equivalent circuit. The formula for fault circulation is derived through double-sided elimination method. Spiral and cage grounding scattering models are constructed employing the finite element method. Moreover, each provided model ground surface potential of working shaft is deduced. The 220 kV submarine tunnel power cable project at Ningbo-Zhoushan in China is considered as a case study by double-sided elimination method. The results reveal that the sheath fault circulation initially rises before subsequently declining, reaching its maximum fault circulation at 136.74 kA within the tunnel system. Furthermore, the cage grounding model demonstrates superior scattering effects when compared to the spiral model. When the number of vertical cage conductors are 6, the grounding potential of working shaft is only 917.64 V, which is an allowable range of human contact and step potential of 955 V. This research provides a new method and a reference for the calculation of fault circulation in tunnel cable system and the grounding scattering optimization method.