Abstract: Fertigation technology plays a crucial role in the fertilizer utilization efficiency and sustainable development of modern agriculture. Among them, the piston-type fertilizer injection pump serves as the vital equipment for the uniform fertilizer injection in the large-scale irrigation systems. In this study, the numerical investigation and experimental validation were presented on the internal flow and external performance of the piston-type fertilizer injection pump during reciprocating motion. A three-dimensional numerical simulation model for the transient flow in the piston fertilizer injection pump was established based on the dynamic mesh and the RNG k-ε turbulence model. The gap between the dynamic mesh boundary and the static boundary was treated by the gap model with zero mass flux into the gap grid. The motion of the valve core in the flow field was analyzed by the six degrees of freedom (6DOF) solver. The external properties of the piston injection pump under different conditions were investigated, and the internal flow field characteristics of the pump under nominal condition were analyzed. The reliability of the numerical simulation model was verified by comparing the predicted flow rate with the experimental and theoretical data. It was found that the maximum relative error between the predicted and the experimental time-averaged flow rates was 3.58% for different strokes. The instantaneous flow rates were obtained from the numerical simulations and theoretical calculations. A high degree of consistency was demonstrated in both magnitude and trend, with the maximum relative error of 3.17%. The high velocity zone was concentrated in the flow channel of the outlet check valve, with the maximum velocity of 7.74 m/s, during the discharge phase of the pump. While a significantly increased pressure observed in the pump chamber with the maximum relative static pressure of 69.1 kPa. In the suction stage of the pump, the high flow velocity area was mainly concentrated in the inlet check valve channel. The maximum flow velocity of 8.05 m/s was found to increase the vacuum degree within the pump, resulting in the minimum relative static pressure of -79.8 kPa. In contrast to the discharge stage, the liquid in the suction process entered the pump cavity through the inlet valve, and then impinged on the wall of the pump chamber, indicating the complex mixing and turbulent structure within the pump cavity. There was a lag behavior at the initial stages of both the discharge and suction during the reciprocating motion of the piston. The formation of backflow was attributed to the delayed closure of the inlet and outlet valve core. A sudden change in the instantaneous flow rate of the fertilizer injection pump was also observed. The time of the sudden change was consistent with the time of valve core closure. The amplitude of the flow rate variations in the inlet and outlet valves at the sudden change point were equivalent. The time-averaged flow rate of the fertigation piston pump showed a positive linear correlation with the ratio of piston stroke (k) and reciprocating frequency (f). The volume coefficient of the fertilizer injection pump decreased with the decrease of the piston stroke, but there was no significant change with the reciprocating frequency of the piston. Meanwhile, the measured volume coefficient of the variable frequency regulation differed by only 2.65 percent point from the rated operating condition at the low flow rate (k=20% and f=10 Hz). In contrast, the volume coefficient of the variable stroke regulation differed by as much as 13.94 percent point. Therefore, the frequency modulation control can be expected to more accurately regulate the flow rate of the piston injection pump. It is an optimal strategy for regulating the fertilizer injection in fertigation systems. The findings can provide a strong reference to optimize the fertigation piston injection pump.