Abstract: Sand filter is considered to be one of the best choice to prevent clogging in micro-irrigation system. Head losses in media filter are produced by the friction loss due to the internal auxiliary elements of the filter container and the resistance offered by the media layer. The underdrain element is a core component in sand filter. To deeply understand the hydraulic behavior of underdrain, the experimental and numerical investigations were conducted to explore the flow resistance characteristics and flow field distribution in the underdrain. Laboratory experiments of filtering and backwashing were conducted in Farmland Irrigation Research Institute, Chinese Academy of Agricultural Sciences, Xinxiang, China. A trapezoidal underdrain element applied commonly in domestic market was selected in the experiment. It was installed in a scaled sand filter based on a commercial filter with 600 mm internal diameter. The sand filter model were tested at eight filtering velocities ranging from 0.010 to 0.040 m/s, and eight backwashing velocities ranging from 0.005 to 0.040 m/s, which were consist with the actual commercial filter operation. The experimental variables were monitored in real time by a data acquisition system consisted of a flowmeter and pressure transmitter. The internal flow behavior through the filter was simulated using the Computational Fluid Dynamics (CFD) method. A CFD model of the underdrain was developed by ANSYS Fluent 15.0. The geometry of the underdrain and its unstructured mesh were designed by ICEM software. Due to the turbulent flow regime in the underdrain and layer, the realizable k-ε turbulence model and the porous medium model were selected for solution. The computing area and the boundary condition of inlet and outlet were proposed. To ensure the reliability of the numerical simulation, the experimental values were compared with the simulated results. The results showed that the simulated values of head loss were in good agreement with the experimental values with the relative errors between 3.54% and 6.53%. The tested results showed that as the filtering velocity increased (>0.024 m/s), the pressure drop produced by the underdrain improved more significantly than the media layer. When the velocity increased to 0.040 m/s, the 72.3% of the pressure drop were produced by the underdrain. At the same filtration velocity, the pressure drop of backwashing mode was higher than filtering mode. The results confirmed that it was important to select appropriate filter model and avoid overload operation for the irrigation system. The simulated pressure and velocity contours showed that the structure without slots at the top of underdrain increased the non-uniformity of the flow streamlines and affected the backwash efficiency. Due to the narrowing of the flow section gradually from the layer to the underdrain outlet, the flow velocity increased drastically. When filtration velocity was 0.030 m/s, the velocities at the narrow slots and the entrance of the collector reached 2.57 and 4.01 m/s, respectively, resulting in significant pressure drop in the two region subsequently. The pressure contours of the collector at different cross sections showed non-uniformity due to the violent disturbance of the flow. In view of these results, a spherical flat underdrain with smaller aspect ratio and more uniform slots distribution was designed. Compared to the trapezoidal underdrain, the distance of the flow through the underdrain decreased by 14% and the passing area increased by 27%. In addition, the diameter of the tube outlet increased to 30 mm. The modification favored vertical flow through the porous medium and the underdrain chamber with a more direct exit, thereby reducing pressure drop. The CFD simulations of the modified underdrain showed the smoother streamline around the underdrain for filtering mode and a smaller vortex scale for backwashing mode, indicating excellent passing capacity and backwash efficiency. The study could guide the design of filter for irrigation equipment manufacturer, irrigation project designer and field operator.