Direct acting pressure regulator is widely used for ensuring high uniformity and security for irrigation system. This research is aimed to investigate the effect of geometric and spring parameters of the pressure regulator on its regulating performance. For this purpose, an improved numerical simulation method was presented to obtain the entire characteristic curves including both the performance line of unregulated segment and performance curve of regulated segment. The modification of partial boundary conditions and calculating process of the dynamic model which was established in a previous study were carried out. The flow rate of pressure regulator calculation domain was controlled by a further downstream orifice along with the appropriate pipe length before and after the orifice, instead of directly setting the flow rate on the regulator outlet boundary. The cross section area of the orifice increased or decreased by its boundary moving till the desired flow rate obtained. The technique of layering dynamic mesh was employed for the movement of the orifice and the regulating plunger boundaries and mesh deformation of surrounding flow domain. The velocity of the orifice boundary, the calculation process of dynamic equilibrium of the regulating plunger, and the spring stiffness were carried out using a special user defined functions developed to couple with ANSYS Fluent code in every time step. Results of numerical simulations under inlet pressure ranging from 0.025 to 0.4 MPa and different flow rate conditions were verified by laboratory measurements. The influence of the geometric and spring parameters of the pressure regulator on its preset pressure and initial regulating pressure was investigated using the validated numerical simulation method by single-factor and five-level experiment design. The results shown that the relative deviation between numerical preset pressure and slope of performance line of unregulated segment and test values lay within-19.3 and 9.4 %, respectively. Numerically predicted performance indexes were in reasonable agreement with those obtained by experimental tests, which verified the reliability and calculation accuracy of the proposed numerical simulation method. The preset pressure dropped nonlinearly as the increase of two parameters affecting the force induced by water pressure difference of the regulating plunger which were the sectional areas of plunger upstream face and downstream face. The preset pressure rose proportionally with the increase of three parameters affecting the spring force that were the height of the flow orifice of the seat, the spring stiffness, and the pre-stressed spring length. The initial regulating pressure reduced with increase in the sectional area of plunger upstream face, the height of the flow orifice of the seat, the spring stiffness, and pre-stressed spring length respectively, whereas with a decrease in the sectional area of plunger downstream face. Effects of geometrical and spring parameters on regulating performance are successfully revealed thought the computational fluid dynamics analysis which forms the theoretical basis for future design and optimization of pressure regulator. [ABSTRACT FROM AUTHOR]