An online detector of pesticide mixing mass concentration was developed based on the fact that different concentrations of solution had different refractive index. A 650 nm diode laser was used as the light source and a PSD as the light spot position detector. A PMMA triangular detector tube and a signal adjustment circuit based on OPA4277 chip were designed and manufactured. The experimental results showed that at the flow of 4.7, 5.4 and 6.1 L/min, the determination coefficient of position/concentration fitted curves were all above or equal to 0.983 1, and it increased with the flow decreased. At the flow of 6.1 L/min, the maximum deviation was 0.075 1 g/L and the average relative error was 8.99%, which showed that the developed detector could be applied to mixing mass concentration online detection.