NH is the most abundant alkaline trace gas in the atmosphere which plays an important role in acid deposition and aerosol formation. The NH on-line monitoring system based on tunable diode laser absorption spectroscopy and long open path technology was used to continuously measure atmospheric NH concentration at an urban site near Beijing National Stadium during Beijing Olympic Games. The monitoring results show that the atmospheric NH concentration variation has an obvious diurnal periodicity, the rule is the concentration decreased in the daytime, and increased to the maximum at night. Moreover, the NH peak concentration decreased at the beginning of Beijing Olympic Games. The results reveal the positive effects of auto restriction for reducing NHconcentration and restricting aerosol formation.