The widelyused online mixing system has a small range of mixture ratio and a detection problem at small flow ratio of pesticide. This paper developed an onlinemixing system, which mixed pesticide into carrier directly. A spiral porous mixer, which included lefthand type and righthand type of the spiral plates, was designed and assembled alternatively. A diskshaped mixer was proposed to promote the uniformity of the concentration from the spiral porous mixer. This system consisted of a precise plunger pump to deliver the pesticide, an embedded controller based on STM32 to detect the water level in the tank, a diskshaped mixer, and an electromagnetic switch valve to provide a certain outflow of water. The water and pesticide were mixed in the spiral porous mixer, then flowed into the discshaped mixer, and sprayed on the crops by the spray pump. The pressure sensor was calibrated with the water level in the water tank, the flowmeter was calibrated with the pressure sensor, and used to test the error. The pulse frequency driving the plunger pump was calibrated with the rotate speed, and used to test the outflow error of pesticide. In the range of the work flow, the flow of the water and the flow of the pesticide were measured at the same time. Using chlorpyrifos as the target pesticide, the sampling points of the mixture concentration were measured by a liquid mass spectra. The actual density of the sampling points was obtained, and a comparison experiment with the artificial full mixing of the pesticide and the water was carried out. The experiment results showed that working at the mixture ratio of 150∶1 ~ 1000∶1, the average flow error of water was 1.35%, the maximum error was 7.15%; the average flow error of pesticide was 2%; the maximum error was 3%; the maximum error of the mixture ratio was 6.75%. After a test of mixing using chlorpyrifos, the average error of the mixture concentrations was 11.7%.