The technique of photoacoustic (PA) spectrum is based on the conversion of photon to acoustic energy by collision quenching of the excited molecules. It holds the characteristic of higher detection sensitivity, wide detection spectral region, no damage to the sample etc. It is used in many scientific observation areas such as gas composition analysis, research on chemistry and biology, environmental monitor and so on. In the present paper, the analytic formula of the PA signal produced from the interaction of intense laser with gas system was deduced by solving the dynamic rate equation about the interaction of photon and material. The results show that the magnitude of the PA signal depends on the factors of molecular absorption cross-section, laser intensity, photon number absorbed by the molecule and collision relaxation rate. With the aid of the relation of the PA signal versus laser intensity, the PA spectrum of NO molecule in the wavelength region of 420.0-470.0 nm is ascribed to the transition of X 2Π(v″=0)→A 2Σ(v′=0,1) and X 2Π(v″=0)→E 2Σ(v′=2, 3, 4), F 2Σ(v′=1,2,3) and R 2Σ(v′=0, 1). These transitions are realized via two or three-photon process. The vibration constants of NO A 2Σ, E 2Σ, F 2Σ and R 2Σ electronic states were calculated from the wavelength of the spectral peaks. They are 2 346, 2 342, 2 397 and 2 381 cm-1 respectively. The results are consistent with the one of other method. The phenomenon of saturation appears when the buffer gas pressure is high enough. This is owing to the finite excited molecules.