The oxidizability of NO3 radical in the nocturnal atmosphere is comparable with that of diurnal OH radical. Given the importance of NO3 radical in the nocturnal chemical process, accurate measurement of its concentration and analysis its nocturnal chemical process have important significance. The article introduces cavity ring-down spectroscopy (CRDS) instrument which is applied to measure atmospheric NO3 radical. Light from a red laser diode (the wavelength is 662 nm and line width is 0.3 nm) is coupled on-axis into an optical cavity formed by a pair of high-reflectivity mirrors (R≥99.998 5%) to achieve an effective absorption path length of approximately 20km. And it researches nocturnal chemical process of NO3 radical in view of the fall and winter heavy traffic areas. The measurement of NO3 radical with cavity ring-down spectroscopy was performed in Beijing from October 29 to November 15, 2014. During the observation, the concentration of NO3 radical is relatively low with the maximum of NO3 radical concentration of 50pptv and the average of its concentration of approximately10 pptv. Combining of NO2, O3 and NO data, the observation results are analyzed. The NO3 production rates ranging from 0.04 to 1.03 pptv·s-1 were calculated throughout the observation, and NO3 lifetime averaged at 68 s. The NO3 loss process in the atmosphere is further analyzed. Combining of related auxiliary data, the influence of different humidity as well as particulate matter concentrates on the atmospheric NO3 removal is researched. When atmospheric RH≥60% and PM2.5 concentration mainly greater than 60 μg·m-3, the correlation coefficient of the logarithmic correlation between NO3 lifetime and NO2 mixing ratio is 0.79, NO3 is mainly removed by the indirect loss process; however, when atmospheric RH≤40% and the concentration of PM2.5 mainly smaller than 60 μg·m-3, because of the observation site is close to national highway and influenced by local pollution source, the direct loss process is main; When atmospheric 40%