At present,one of the urgent problems need to solve is the motor vehicle causes to urban air pollution. In order to analyse the effect of the motor vehicle contribute to the urban air pollution of Beijing,the particulate mass characteristic over Jianxiang bridge was monitored continually with lidar in August of 2007. The representative data which was monitored during Beijing to rehearse olympics traffic control was discussed. The system structure and technical parameters of Mie lidar were presented and the data-processing method was discussed,the particulate mass concentration was retrieved. The particulate mass concentrations with the traffic volume were compared and black carbon (BC) concentrations which were measured in the similar meteorologic condition during traffic uncontrolled. The results indicated that the particulate mass concentration was falling,and there was a good correlation between the particulate mass concentrations and the traffic volume. All these proved that the program of traffic controlled improved the quality of the city air.