A new type of organic polymer nonlinear optical compound filmPolypyrrylarylenemethine/polyvinyl is prepared. In the Z scan experiment, two different types of concentric diffraction rings are observed in the far field depending on the position of the sample relative to the Gaussian beam waist. This phenomenon can be explained by the diffraction effect of the Gaussian beam from the laser induced phase hole in the nonlinear medium caused by Kerr effect and thermo optic effect. Based on Fresnel Kirchhoff diffraction integral equation, a mathematical model on the diffraction of nonlinear optical media to Gaussian beam is established. The numberical calculations give good agreement with the experimental results.