Atmospheric refractivity has a significant impact on measurement precision in the field of photoelectric detecting. To improve the detecting accuracy of photoelectric fields, a pure rotational Raman lidar technique for detecting tropospheric refractive index is discussed in detail. Higher-lower isolating two portions of the pure rotational Raman backscattering spectrum of N2 and O2 are obtained by separating from other singles using the double grating monochromator. By caculating ratio of the two return signals, atmospheric temperature and pressure profiles are derived, as well as the measurements of air refractivity. Compared with the theoretical model, the accuracy for detecting atmospheric refractivity using the pure rotational Raman lidar is convincingly illustrated by the results of laboratory experiments. Several atmospheric refractive profiles are shown and the characteristic of refractivity is acquired at different times and on different days. The results show that the changes of the refractivity are little during the night and reach only 0.4% below 7.5 km. The fluctuation of the refractivity between different days is obvious and reaches 3.5% below 4.5 km.