In order to detect the pesticide residues quickly in farm products, a rapid detection system was developed to determine the pesticide residues in agricultural products based on the inhibition of organophosphates against acrtyl-cholinesterase. The system mainly consists of a solid light, a sample container to store the liquid, a micro-controller based testing circuit and a personal computer. The amount of pesticide residues was determined by measuring the light absorbency of the sample liquid after inhibition reaction. The changes of light absorbency with reacting time, the influence of temperature on the activity of enzyme, and the relationship between the concentration of pesticide residues and the inhibition rate of enzyme were investigated. The results show that the light absorbency increases with reacting time, and the increment is much higher in initial reacting period; the enzyme has good performance at the temperature of 20～40℃; the inhibition rate of pesticide residues on the activity of enzyme is distinct, and it can be determined that the pesticide residues exceed the normal level when the inhibition rate exceeds 70 percent.