A new type of π-conjugated polymer―Polypyrrylarylenemethine is designed and synthesized, which shows large nonlinear optical characteristics. It is discovered in experiment that as the 526 nm TEM00 laser beam passing through the Polypyrrylarylenemethine/N-Methyl-2-pyrrolidone (NMP) solution, numbers of concentric diffraction rings arise in far field. This phenomenon can be explained by the spatial self-phase modulation of the Gaussian beam passing through the nonlinear medium. With theoretical analysis and numeric calculation, it is found that the transverse nonlinear phase shift caused by the sample can be very well fitted by a Gaussian function. The nonlinear refractive index n2 can be estimated according to the number of the diffraction r ings. The n2 of Polypyrrylarylenemethine/NMP solution is about -1.85×10 -3 esu.