Small flow high precision real-time dynamic detection and control of pesticide is a key problem which need to solve urgently for on-line mixing device. Flow rate passing the flow regulating valve is related to differential pressure between the upstream and downstream of the valve, fluid density and the valve opening. By developing the relation expression between flow rate and these three variables, real-time detection and control for pesticide flow rate can be implemented using the control valve. In order to meter and control the flow rate of water and pesticide by using flow meter and control valve respectively, the online mixing pesticide device is designed. The calibration method of control valve is proposed and the flow rate relation expressions are developed. The experiments are conducted for the designed mixing device. The experimental results show that the relative error of the pesticide flow rate is less than 4% as the pesticide flow rate is between 24mL/min and 240mL/min.