In order to study the dynamic diffraction characteristics and grating profile of phase gratings in liquid crystals (NLC), a new method used for recording the dynamic phase grating with an asymmetric profile in C₆₀ doped homeotropically aligned nematic liquid crystal (NLC) is presented, the generation mechanism of grating is analyzed and the mathematical model of asymmetric profile is established. First, an oblique incidence parallel beams is used to record thin asymmetric dynamic phase holographic grating. Then, a beam of light is used as a probe to read out the grating and record the diffraction intensity with a photodetector. Experimental indicates that the maximum diffraction efficiency is more than 60%, and the diffraction intensity distribution is asymmetrical. Based on the characteristics of asymmetric diffraction distribution, the physical mechanism of producing asymmetric diffraction distribution and diffraction efficiency exceeding the theoretical limit of Raman-Nath diffraction of symmetric gratings is proposed based on photorefractive effect. Finally, the hypothesis of a grating with asymmetrical fringe close to saw-tooth profile formed in the NLC is given, and the mathematical model of describing asymmetric phase gratings is proposed. The results indicate that the high diffraction efficiency of the phase grating originates from the non-sinusoidal modulated space charge field in the liquid crystal. The asymmetric electric field is mainly the result of surface charge modulation and Carr-Helfrich effect. The proposed model of asymmetric saw-tooth profile grating can well describe the reason why the asymmetric diffraction and diffraction efficiency exceed the theoretical limit of symmetric profile gratings of Raman-Nath diffraction theory.