Lightning weather is frequently accompanied by heavy rainfall, resulting in the bridging of the insulator umbrella skirt of the transmission line by rain columns. Under the impact of lightning, the "bridging effect" may occur. Simultaneously, under the influence of multiple factors, the internal insulation of porcelain insulators deteriorates, forming zero-value insulators and further reducing the insulation performance. In this paper, 35 kV porcelain insulators on transmission lines with an operating life exceeding 15 years are selected, and tests on the lightning impulse discharge characteristics under heavy rainfall conditions in different zero-value positions are conducted. The research indicates that for the porcelain insulator string, whether with or without zero-value insulators, the rain flashover voltage decreases as a power function with the increase of rainfall intensity and the conductivity of rainwater. The rain flashover voltage of the intact insulator string is more significantly affected by rainfall intensity and the conductivity of rainwater. When the zero-value insulator is in different positions, the rain flashover voltage gradually decreases with the increase of rainfall intensity and the conductivity of rainwater, and a saturation trend is observed. When the zero-value insulator is located at the high-voltage end, the rain flashover voltage is the highest, and the influence of rainfall intensity and the conductivity of rainwater are significant. In this paper, the parameters of the rain shield are optimized. After its arrangement, the rain flashover voltage significantly increased by an average of 100.2%, and a clear saturation trend is presented with the increase of the outer diameter. Before and after the installation of the rain-shedding shields, obvious changes occurr in the development path of the electric arc, and the development time is significantly shortened. Compared to the condition before installing the rain-shedding shields, the arc length decreased by 39.99% and the arc propagation speed increased by 180.02% after installation.