Nitric oxide(NO) is one of the most important gas species in hypersonic vehicle′s flow field，and has significant influence on infrared radiation transferring. Therefore, it′s necessary to acquire an efficient algorithm to calculate NO′s equivalent band width. According to HITEMP 2010, the distribution characters of spectral lines and absorption coefficients of NO in infrared spectrum 1-15 μm were studied. Then an improved k-distribution model was proposed and the equivalent band width was solved by applying Gauss numerical scheme. Besides, the effects of absorption coefficient thresh on both g-k curve and equivalent band width were researched and the criteria of selecting the thresh was put forth. Experiments show that the results of the proposed algorithm agree with the ones obtained by line-by-line calculation, and the relative errors were less than 5%. When compared with the wide band k-distribution algorithm, the proposed algorithm has better computing efficiency and moreover it has better accuracy while using a proper thresh.