A pesticide mixing ratio detection unit based on light permeability of pesticides, a pesticide injection device controlled by PWM duty cycleand a low power consumption control unit based on single chip microcomputer MSP430F149 were designed. A performance test and a calibrating experiment were conducted for each unit. An online pesticide mixer with mixing ratio feedback was constructed and a fixed-point tracking experiment on the Paraquat SL mixing pesticide and water by 1∶200 was conducted. The result showed that the online pesticide mixer developed by the method of online mixing pesticide with mixing ratio feedback based on light permeability of pesticides could acquire mixing ratio information in time, automatically adjust injection volume of pesticides, accurately track the set of mixing ratio, and effectively meet the requirement of online mixing pesticides. 