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An optical sensor system composed of a gas-testing chamber and a liquid-sample chamber and an automatic sampling unit was prepared for rapid field detection of ammonia nitrogen in water based on reversible color change of the sensing element induced by ammonia gas released from the alkalinized water sample.The gas-testing chamber including a 625-nm LED,a photodetector and a sensing element inside was connected with the sample chamber in a gas -proof manner via a gas mini-pump.The sensing element is a transparent porous glass fiber membrane in which the NH3-sensing reagent was loaded.The program-controlled automatic sampling system consists of a diaphragm liquid pump whose outlet was connected with a 3-port solenoid valve and whose inlet was linked with 4 parallel-connected 2-port solenoid valves.The sensor system is compact and easy to use,able to work in a high-humidity atmosphere and possessing a high sensitivity and a good repeatability and a long lifetime.Its dynamic range is from 0.1 mg/L nitrogen (below Class-I water) to 10 mg/L (beyond Class-Ⅴ water).The time required for one complete measurement with the sensor is about 60 min.