[Objective] Aiming at the problems that the grounding shield layer of three-phase insulated tube-type busbars is prone to floating or degradation faults during operation, which further threatens equipment insulation and personal safety, this paper intends to deeply reveal the potential evolution mechanism under such faults and provide theoretical support for the analysis of electric field characteristics under fault conditions. [Methods] First, a field-circuit coupled finite element model of 35 kV three-phase insulated tube-type busbar was established,and the electric field distortion characteristics and potential distribution under the shield floating state were analyzed. Meanwhile, a three-phase coupled equivalent circuit model considering distributed parameters was constructed to calculate the variation law of the floating potential of the shielding layer with respect to the stray capacitance to ground and the grounding resistance. Furthermore, combined with a 1 km line model in practical engineering, the influence of distributed parameters on the induced voltage was analyzed. [Results] The results show that, compared with normal operating conditions, the surface potential of the faulty phase rises with the conductor under the floating shield layer, and electric field distortion occurred in the interphase air domain. The main insulation capacitance was significantly larger than the stray capacitance to ground, causing the shield layer to lose its potential clamping function. Under the resistance grounding condition where the shield layer is grounded through a resistor, the accumulation of displacement current caused by distributed parameters made long-distance busbar more sensitive to changes in grounding resistance. [Conclusion] Floating of the grounding shield layer results in a sustained and stable high-voltage state, and the associated electric field distortion severely damages the full shielding characteristic of insulated tube-type busbars. To ensure personal safety and equipment insulation safety, based on the parametric analytical model derived in this paper for estimating the safety threshold of grounding resistance, it is recommended that the grounding resistance of 1 km long insulated tube-type busbars at the 35 kV voltage level be controlled within 12.3 Ω to maintain sufficient safety margin.