The spatiotemporal characteristics of bridge deck runoff under a natural rainfall event are explored. The Taizhou Bridge is taken as a study case, and a hydrodynamic model based on the two-dimensional shallow water equations is used to analyze the runoff characteristics. The results indicate that the runoff velocity rate and depth are positively related to rainfall intensity, yet they have different response degrees to it. The inlet's effect degree on lane water film has a positive relationship with rainfall intensity. A natural logarithm function ( R2 = 0. 706 ) can illustrate this relationship. However, the inlet's effect degree on ponding at the curb shows a negative relationship with the rainfall intensity. A negative exponential function ( R2 =0. 824 ) can reveal this relationship. With the decrease in the longitudinal slope SL , the ponding depth at the curb increases significantly at the bridge approach slab, whereas the lane water film thickness ( WFT ) is almost unchanged, but the lane WFT increases greatly at the location with the minimum longitudinal slope. It is concluded that the characteristics of the bridge deck runoff present apparent spatiotemporal differences, the inlet's effects on bridge deck runoff are quantitatively correlated with rainfall intensity, and the effective drainage measures are necessary for the bridge approach slab.