The present invention discloses a node flow optimization distribution method for improving the accuracy of transient hydraulic simulation of a water supply in-series pipeline. The present invention optimizes the flow distribution coefficients of intermediate nodes to minimize the impact thereof on the calculation and analysis of transient flow. Further, the simplified error generated by the node flow distribution can be quantified and evaluated by the control threshold of the simplified errors to achieve effective control of the simplified process. In addition, the simplified operation of the method of the present invention is carried out sequentially from the intermediate node with the smallest simplified error, which effectively overcomes the potential defect of the conventional node flow distribution that leads to a significant reduction in the accuracy of the model, and can ensure the reliability and accuracy of the simplified operation of the same-diameter in-series pipeline.