NO and NO2 are the common gases and have serious harmfulness to the atmosphere pollution of environment. To detect the concentration of the two gases in pollution, we construct a low cost photoacoustic spectrum gas measurement system based on the thermal radiation light source. Absorption lines of the NO and NO2 between 2500 and 6667 nm are calculated. By modeling the photoacoustic transmission line the relations between quality factor, acoustic pressure and cavity length, cavity radius, modulation frequency are obtained, and the design of geometric construction of photoacoustic cell is also guided. The experiment show that there exists a good linearity between photoacoutic signal detected in the system and gas concentration, and the system ultimate detection sensitivities to the NO and NO2 are 4.01 and 1.07 μL/L respectively. When the emission wavelength of laser is regulated suitably and the edmund optics is chosen reasonably, this system is also suitable for the concentration-detection of other trace gas.