Self-phase modulation phenomena was found in 5CB liquid crystal, which accounted for the far field diffraction ring phenomena induced by high power laser traversing thin liquid crystal cell. Experimental and numerical methods were employed to study the mechanism of the self-phase modulation in 5CB. Experimental results indicate that diffraction ring phenomena can be found when the incident laser intensity rise to 10 kW/cm2 and 60 W/cm2 for 532 nm and 1 064 nm laser separately, but will disappear when the effect of laser interaction with liquid crystal sustained hundreds of mili-second. Numerical results applying Kirhoff diffraction integral formula were in good agreement with experimental results. Based on thermal conductivity theory, the relationship of the third order nonlinearity with the incident laser spot and wavelength in 5CB liquid crystal was discussed qualitatively. It was indicated that thermal effects take the main role in the nonlinear self-phase modulation phenomena which is greatly associated with the absorption coefficient and the laser spot diameter.