Crude oil leaking in rivers may cause serious damages, such as environmental pollution and death of river animals. The crude oil spreading in rivers could be much faster than that on lands, as the leaking crude oil may flow rapidly downstream with the flowing water. Therefore, accurately estimating the transient crude oil spreading area in rivers is a vital task for emergency response and disaster rescue. However, the estimating methods in the literature mainly refer to spreading process in underwater, ocean and soil, which commonly happened in the history. The crude oil transportation pipelines in China pass through many rivers with very complex channel geometries, introducing the necessity of estimating of leaking crude oil spreading in actual river channels. In the current study, the crude oil spreading process along an actual river channel is numerically simulated. The river channel geometry is extracted from a map database, which is further treated using image binarization and edge extraction to obtain the discrete river channel data. The river channel data is then smoothed by picking less data representing main geometric characteristics. The smoothed data is used to reconstruct the river geometry and generate calculation mesh. The mesh is a two-dimensional structured grid with several possible leaking points along the actual crude oil transportation pipeline passing through the river. A multi-fluid MIXTURE model is used to simulate the crude oil spreading process on the water surface, meaning crude oil blending in the flowing water. Cases with a leaking mass flow rate of 240 kg/s and a river flow velocity of 1.58 m/s are simulated for 10 possible leaking sources on the traversing pipeline. The effect of leaking locations and river channel bending on crude oil spreading on river surface were significant according to the simulation results. Sudden widening river channel may result in vortexes and slightly delays the crude oil spreading. The simulated data could be used to make the rescue strategy of crude oil leaking in this specific river.