A nitrogen dioxide (NO2) detecting system based on laser internal modulation and differential detection was designed in order to real-time monitor NO2 concentration in the air.According to the characteristics of NO2 absorption peak，the blue laser operating at 450nm was selected as the light source.The internal modulation technology was used，and the output light intensity and frequency were controlled through modulating the drive current by TTL (Transistor-Transistor Logic).The NO2 concentration was measured at room temperature and atmospheric pressure，and experimental data were processed with the least square method.The results show that the detection limit of the system is 6ppm and a fitting coefficient is 0.9996.