A compact optical sensor for the automatic detection of ammonia nitrogen in water was developed based on a combination of liquid-gas separation and gas-phase reversible colorimetry. The sensor consists of a sample chamber, a test chamber, a program-controlled sampling unit and a data-processing circuit module. The sensor performance was investigated by using a series of aqueous ammonium chloride solutions with different concentrations as the ammonia nitrogen samples. The linear relationship between the measured absorbance and the ammonia-nitrogen concentration was obtained and the high reliability of sensor was demonstrated by comparing the measured and given concentrations of standard ammonia nitrogen samples. The experimental results indicate that the sensor works continuously and automatically for a long time at a room temperature and has good repeatability. The detection limit is 0.04 mg/L (NH3-N). The sensor is characterized by its smaller volume, light-weight, lower cost and power consumption, high-level automation, and it is suitable for on-site rapid detection of ammonia nitrogen in a practical water environment.