Sprinkler irrigation is one of the main water-saving irrigation methods at home and abroad. As a common water-saving irrigation equipment, impact sprinkler is widely used in landscaping and agricultural irrigation fields. The traditional impact sprinkler with circle spray filed has the disadvantages of overlap spray, excessive spray and missing spray. In response to above disadvantages, impact sprinkler with square spray field has been improved but it remains facing many problems such as complicated mechanical structure, excessive head loss, and it’s spray shape deviates from square. In order to reduce the local head loss, the inner passageway of spray’s flow regulator consist of hollow shaft and threaded pipe was optimized with streamline. To achieve the goals of accurate square spray field, the docking port between hollow shaft and threaded pipe was reshaped with diamond shape as entry end connect to threaded pipe and star shape as export end connect to hollow shaft. The improvement above can effectively reduced the head loss that was unfavorable to the spray shape, and improved the range and spray shape factor. To evaluate the drag reduction effect in passageway of sprinkler, the flow regulator was divided into four part according to it’s inner flow state. Then the head loss formulas for each part were derived by energy equation and momentum equation of hydrodynamics. The whole head loss of the optimized flow regulator was expressed as sum of the four parts head loss related to their flow velocity and loss coefficient effected by passageway’s shape at last. The flow velocity of each part could be obtained by simulating which was conducted in this paper. This work provided a theoretical basis for the selection of the flow regulator inlet and outlet and flow passage optimization of the square spray field impact sprinkler. For global flow analysis and provide flow velocity result for head loss calculation, the three-dimensional unsteady numerical simulation of the inner flow path of the square spray field impact sprinkler was carried out. The 3D geometric model of flow regulator was established using Pro/E software. The distribution regularities of velocity and pressure were simulated by FLUENT software with the RNG k-ε turbulence model. Pressure inlet was set as the inlet boundary condition in the model, with the working pressure 250, 300, 350, 400, 450 and 500 kPa respectively. According to the relationship between the flow rate and the range of the same type of Rain Bird impact sprinkler without the flow regulator, the range of four corners of a square spray field sprinkler with optimized flow regulator was calculated. According to the working equation of the square spray impact sprinkler, the variation rule of the range under different rotation angles can be obtained. Finally, comparting with the same type of Rain Bird sprinkler, the optimization effect of the square sprinkler was evaluated by the variation of the head loss, flow rates, range change and spray shape, and the reliability of the results was verified. The results show that the new type of square spray field sprinkler is simple in structure, hydraulic automatic control, energy saving, durable and convenience, and has standard spraying shape with small head loss. Compared with the square spray field sprinkler before optimized, the head loss of the optimized square spray field sprinkler in the rated working pressure reduced by 25.8%. The flow regulator range of the improved sprinkler is enlarged, with maximum flow rate increasing by 12.87% and minimum decreasing by 12.83% respectively. In addition, the spray shape factor is 86.89% which rises 15.6 percentage points and the excessive spray error reduces 9.85 percentage paints. This provides a theoretical reference for the design, manufacture and application of the square spray field sprinkler.