In this paper, a numerical simulation model of the flow field inside the diaphragm valve is established based on the k-ε turbulence model (where k represents the turbulent kinetic energy and ε represents the turbulent dissipation rate). The mass flow at the inlet and static pressure at the outlet are used as the numerical calculation conditions, and the accuracy of the model is verified through experiments. Based on this, the model is applied to analyze the internal flow characteristics and pressure field distribution of the valve body under different import flow rates (2.787-33.273 kg/s), and an accurate quantitative relationship between import flow rate and valve body head loss is established. The results show that: 1 the numerical simulation can better predict the head loss of the valve body under different flow conditions. When the inlet flow numbers are 5.546, 11.091 and 16.637 kg/s respectively, the relative errors between the experimental and numerical simulations are only -6.433%, 4.619%, and 7.264%. 3 With a constant inlet flow, the static pressure decreases from inlet to outlet in flow passage. Flow contraction caused by the blockage of the valve wall and the flow impinging on the diaphragm creates a large static pressure gradient in the valve body. 4 After water passes through the narrow flow channel on the valve wall, a cavitation zone and reflux phenomena occur downstream of the valve body. The cavitation zone appears mainly in the 1/3 fluid domain away from the outlet. As the inlet flow increases, the vortex in the downstream of the valve body intensifies and the reflux phenomenon becomes more significant, but the scope of the reflux zone does not increase significantly. 2 After verifying the accuracy of the model, this paper studies the valve body's internal flow characteristics and pressure field distribution under 18 different inlet flow conditions. The model establishes a relationship between the inlet mass flow Q and the head loss ΔP of the valve body. For Reynolds numbers from 37 927-21 5984 and inlet mass flow Q from 2.787-15.428 kg/s, the fitting equation is ΔP = 2 076.31Q - 7 567.49 (R² = 0.964). For Reynolds numbers from 240 097-467 009 and inlet mass flow Q from 17.141-33.273 kg/s, the fitting equation is ΔP = 5 688.02Q - 67 317.39(R² = 0.993). These results are useful for hydraulic calculations in irrigation systems. [ABSTRACT FROM AUTHOR]