A new method was presented to determine lidar geometric form factor. The intensity distribution charateristic of pure rotational Raman spectra at different temperature is studied. Lidar geometric form factor can be determined accurately with the sum of pure rotational Raman lidar signal and the Rayleigh-Mie lidar signal of atmospheric molecules. The method reduces aerosol wavelength exponent effect compared with vibrational Raman method and aerosol uniformity limit in contrast to horizontal determination method. The numerical simulation shows that the maximum relative error of geometric form factor retrieved from three kinds of temperature distribution modes and three kinds of aerosol wavelength exponent distribution modes is less than 0.5%. The lidar deomenic form factor is determined with real lidar aignal.