Differential optical absorption spectroscopy (DOAS) technology can accurately and real-time monitor the concentration of flue gas components, which is an effective means to control the emission of flue gas pollutants from the source. In DOAS system, the change of light source intensity is an important factor affecting the long-term reliability of measurement. In order to solve this problem, a new DOAS retrieval algorithm without calibration of light intensity is proposed in this paper. The algorithm uses the neighbouring wide-band cross section to normalize the narrow-band cross section of gas characteristic absorption, so as to obtain the equivalent absorption intensity parameter, αeq(λk), which is not affected by the change of light source intensity. By comparing the measured value of this parameter with the calculated value of standard cross section, the gas concentration can be calculated. Compared with the traditional algorithm, this algorithm does not need complex calculation such as polynomial fitting and signal filtering, and it is more convenient for hardware implementation. The measurement system was built, and N2 and high concentration NO standard gas were used to prepare different concentrations of NO. The measurement results show that the characteristic absorption of NO only appears in 195.5 nm (absorption 1), 204.7 nm (absorption 2), 214.8 nm (absorption 3), 226.2 nm (absorption 4), and the full width at half maximum of the four absorptions are about 1 nm. The linear regression coefficient R2 of characteristic absorption 4 (226.2 nm) reaches the maximum of 0.998 17, which verifies the feasibility of the new method.