In-situ monitoring ammonia volatilization is needed to improve nitrogen fertilizer utilization efficiency, reduce the waste of resources, and solve eco-environmental problems. Open multi-path NH3 monitoring system has been developed based on tunable diode laser absorption spectroscopy (TDLAS) technology combining with long open-path technique, wavelength modulation technique, fiber sensing technique and time division multiplexing technique, which are used for ammonia concentration in-situ detection after farmland fertilization at Fengqiu experimental station of agro-ecology, Chinese academy of sciences. Monitoring results show that the variation of ammonia concentration has an obvious diurnal periodicity, the general characteristic of diurnal variation shows that the concentration is high in the daytime and is low at night. Ammonia concentration reaches the maximum after a week for fertilization, and then decreases gradually. This system detects gas concentration with large range, high sensitivity, short response time without gas sampling. Thus, it is an effective method for ammonia volatilization monitoring in farmland environment.