To reduce the probability of breakdown due to the gap between the water column formed by the seepage of water from tunnel crown and the cantilever insulator, and improve the stability of the power supply system in the tunnel, a seepage deflector structure that can change the location of seepage water falling from the crown is designed, and its effect on the electric field near the cantilever insulator is analyzed. A 1∶1 three-dimensional model containing crown, water column, and cantilever insulator is constructed and its potential and electric field distributions are calculated, thus examining the effects of water column morphology, water column length, water column position, and inflow channel structure on the spatial electric field distribution in the vicinity of the insulator. The results reveal the following: (1) When the water column is grounded at one end and intermittent, its maximum air gap electric field distortion rate is less than that when the water column channel is continuous. (2) When a continuous column of water extending downwards from the crown drops 68 mm from the high-voltage end of the cantilever insulator, the average electric field strength of the remaining air gap is less than 5.66 kV/cm. (3) The closer the water column is to the low voltage end of the insulator, the smaller the distortion rate of the electric field strength on the surface of the water column. (4) The average electric field strength of the air gap from the crown to the high-voltage fittings is lower than 5.23 kV/cm after the addition of a deflector channel to change the drop point of the water column.