Liquid fertilizer has the advantages of easy absorption, high efficiency and low environmental pollution compared with granular fertilizer. To achieve precise application of liquid fertilizer, a detecting seeds and target-spot application system of liquid fertilizer was proposed. The system needed liquid fertilizer to meet the requirements of small pressure fluctuations during high-frequency intermittent spraying, and ensure pressure stability when the intermittent spraying frequency was changed. Thus, a high-frequency intermittent fertilizer supply system was designed based on PID algorithm. Through the study of the hardware circuit and software program of the fertilizer supply system, the collection and control of the pressure for the fertilizer supply system was realized. And the interactive interface was designed to facilitate the setting of operating parameters and the display of real-time operating conditions. In order to get the best parameters of the PID algorithm, the critical proportion method for parameters tuning was used. After actual experimental adjustment, the proportional, integral and differential coefficients of the PID algorithm were determined to be 4, 0.079 and 0.012. A comparative test was implemented to compare the pressure change of the fertilizer supply system with and without the PID algorithm. The results showed that the pressure of the fertilizer supply system with the PID algorithm was more stable. To evaluate the stability of the fertilizer supply system based on PID algorithm at the operating speeds of 3 km/h, 4 km/h and 5km/h and the setting pressure of 0.10MPa, 0.15MPa, 0.20MPa,0.25MPa and 0.30MPa, a bench test was carried out. The test indicators were maximum deviation, average deviation and standard deviation of the pressure for the fertilizer supply system pressure. The test results indicated that the maximum deviation, average deviation and standard deviation were not more than 6.49%, 1.54% and 0.40MPa, which satisfied the requirements of detecting seeds and target-spot application system of liquid fertilizer for pressure stability.