The fiber-optic biosensor consisted of tubular enzyme reactor and two sensing parts was developed for the detection of organophosphorus compounds in a contaminated water. The organophosphorus compounds was measured by acetylcholinesterase entrapped by Ca-alginate gel on a silicon tube. The litmus dye was used as the indicator of color change due to the inhibition of organophosphorus compounds on the acetylcholinesterase. Transmittance change of light at the input and output parts of tubular enzyme reactor was detected. The biosensor has the linear analytical range of 0-1.0 ppm with response time of 5 minutes. The proposed kinetics for irreversible inhibition of organophosphorus compounds on AChE agreed well with the experimental data. The theoretical model of diffusion and reaction in enzyme membrane was presented to analyze the response of sensor signal. Based on the simulation results using the model, the optimal amount of enzyme loaded and substrate was obtained as 80 Ռg and 6 mM, respectively.