To solve problems in the formation mechanism of water flow in automotive water tank and its complex working conditions, this paper constructed a theoretical model for the uncertainty of water jet in automotive water tank. Then, based on the secondary development characteristics of FLUENT software, the uncertainty model, and the fluid field simulation module, this paper studied the wall effect of the fluid field of water jet in automotive water tank, and obtained fluid fields of single-nozzle and double-nozzle water jet and the corresponding distribution laws of the wall effect. The research results showed that the wall effect had a great influence on the direction of the water jet and the pressure on the wall surface of the water tank, while its influence on the velocity of the water jet was relatively small; the two streams of water jet converged, which was basically consistent with the laws of the wall effect in the water tank; the smaller the distance between the two water jet streams, the more likely the two streams would converge. The research in this paper provided theoretical basis and reliable technical support for the analysis of the formation mechanism of the water flow in automotive water tank, the optimization of the water tank structure and the application of water tank in various industries.