In order to analyze the flow characteristics of the plane S-type axial extension tubular pump unit, the software CFX was used to conduct three-dimensional full channel numerical simulation, and the energy gradient theory was used to analyze the flow characteristics of pump unit under different working conditions. The results show that under different flow conditions, the water flow in the inlet passage of the plane S-type axial extension tubular pump is very smooth, and the overall difference is small. The flow pattern in the outlet channel is quite different, and the smaller the flow rate is, the worse the flow pattern in the outlet channel is. The pressure pulsation at the same position in the plane S-type axial extension tubular pump unit is the largest under low flow conditions. The difference of energy gradient in the outlet channel is the largest under the condition of small flow, which indicates that a large energy loss will occur in the outlet channel under this condition. When designing the outlet passage of the plane S-type axial extension tubular pump unit, the length of the smooth section of the outlet passage should be increased as much as possible to ensure the uniform distribution of energy gradient. The research results can provide theoretical guidance for better understanding of the plane S-type axial extension pump device and optimization design.