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This paper carried out the absorbance experiments on different pesticide formulations and mixed pesticide concentration sensor design for online mixing system based on the precision spray requirements. There are many different pesticide formorlations, among which wettable powder (WP) and emulsifiable concentrates (EC) are most widely used ones in China. The optical methods were chosen for quick and precise measurement based on the physical characteristics of commonly used pesticide formulations and generally low spraying concentrations. Firstly we carried out monochromatic light absorbance experiments to study the optical properties of different pesticide formulations. We prepared the solutions with different possible spraying concentrations of WP and EC of several commonly used pesticdes and used UV-Vis spectrophotometer to measur the absorbance of the prepared solutions. The experiment results show that: in the UV-visible wavelength range, the absorbances of the suspension or emulsion of WP or EC generally decrease with increasing optical wavelength, but the solution concentration and the absorbance value is not proportional. In the 600-800nm range the absorbances have good differentiation. In order to select the appropriate absorbance range of wavelengths and considering the available of laser diode, the monochromatic light with wavelength of 650nm was chosen as the source light. Then we carried out the preliminary design of mixed pesticide concentration sensor. The sensor is composed of monochromatic light source module, the light detection circuit and signal processing module. The structure of the sensor was also preliminarily designed.