Nitrogen oxide, being one of an important trace gas in the atmosphere, may affect the oxidation of the atmosphere, harm the physiological health of the human beings and the animals, and cause photochemical smog, haze, acid depositions and other environmental problems. In recent years, with the rapid development of the economy and the continuous increase of energy consumption in our country, the nitrogen oxide emissions have been remaining at a high level. Therefore, it is of great significance to study the content and chemical properties of the nitrogen oxides in the atmosphere. The methods for determining NOx has tended to become diversified. However, the methods for determining the total reactive nitrogen oxide (NOy) have always been dominated by the catalytic conversion chemiluminescence (CL). In this paper, a method of simultaneous measurement of NO2 and NOy concentration in the ambient air by the thermal dissociation cavity ring down spectrometer (TD-CRDS) is introduced. The performance of the pyrolysis device was optimized, the effective absorption cross section of NO2 was determined, the interferences which may possibly exist in the system (H2O, glyoxal, NH3, N2O and etc. ) was analyzed, and the detection limit (NO2 chamber: 8.72×108 molecules·cm-3; NOy chamber: 9.71×108 molecules·cm-3) and the errors (NO2 measurement: 5%; NOy measurement: 12%) were discussed. In order to verify the performance of the system, the concentration of NO2 was determined by comparing the CRDS and the long-path differential optical absorption spectroscopy (LP-DOAS) synchronously, with a linear correlation factor r=0.960. The concentration of NOy in the atmosphere was determined through comparing with the model Model 42i-NOy analyzer synchronously, with a linear correlation factor r=0.968. Both consistencies is good. A field experiment was performed for a week at Hefei Science Island. During the measurement, the average concentrations of NO2 and NOy were 4.11×1012 molecules·cm-3 and 7.73×1012 molecules·cm-3, respectively. According to the average daily variation diagram, it is found that there is a similar trend in the concentration of NO2 and NOy, and it usually starts to decline at 10:00 and the lowest value occurs at 15:00. Because of its high sensitivity and high time resolution, the CRDS has become a new and simple method for determining the total reactive nitrogen oxide in the ambient air.