This study is conducted to trace the source of sudden river pollution. Specifically, the coupling relationship between forward and reverse mass probability density is used to realize the decoupling of the location, discharge time, and discharge intensity of pollution sources; then, given the one-dimensional water body diffusion model and the monitoring data from the tracer experiment in Truckee River of the United States, a method for tracing the source of sudden river pollution is established on the basis of the improved firefly algorithm (FA). In the method, the monitoring data are divided into a training set and an experimental set, and by the training set data, the improved FA is employed to adjust the hydrological parameters of the river. Then, the adjusted hydrological parameters are used in the experimental set, and data from different monitoring sections are used independently for solutions. Finally, the results are analyzed by variance to exclude the data with large source-tracing errors. The results reveal that the source-tracing results have high accuracy and the ability to correct the monitoring data, which is of certain guiding significance for the actual sudden river pollution.