Abstract: Bridges are usually located at the junction of land and water, where the surrounding area is open terrain, making them to be easily damaged by lightning strikes. The quantitative analysis of lightning transient characteristics and the impulse effect on bridges can provide scientific data to support the lightning protection design for bridges. In this study, a single-tower cable-stayed bridge is taken as a representative case. The CDEGS (Current Distribution Electromagnetic Interference Grounding and Soil Structure Analysis) software is used to establish a three-dimensional simulation model of the bridge. With this model, the magnetic field, step voltage, and lightning current distribution on the tower top, stay cables, and bridge deck under the most severe direct lightning strike scenario typical for cable-stayed structures are simulated. The results are as follows: (1) when the stay cables are struck by lightning, the peak of the magnetic field intensity is highest at the location of electronic information equipment, followed by that at the top of the tower, and the lowest at the bridge deck. The peak step voltage at the ground below the bridge is the largest when lightning strikes the cable, and that is the smallest when lightning strikes the bridge deck. (2) The magnitude distribution of the lightning current on the stay cables is related to the strike location, the distance between the strike point and the grounding system, and the length of the stay cables. When the stay cables are closer to the grounding system and the length is shorter, the lightning current that flows through them is larger. (3) The magnitude distribution of the lightning current on the grounding system is associated with the location of the strike and the position of the grounding system. The closer the grounding system is to the strike point, the larger the amplitude of the lightning current on the grounding system. The grounding system positioned in the middle significantly reduces the lightning current due to the shielding effect. The grounding system located at the edge shows minimal variation in the time domain characteristics of the lightning current, resulting in less reduction of the initial steepness of the lightning wave. However, the grounding system at the middle position experiences a significant increase in the temporal characteristics of lightning current, reducing the hazards caused by the steepness of the original lightning wave.