A new type of optical storing material, the film sample of azobenzene disperse red polymer is measured by using the single beam Z-scan technique. The experimental results show that the nonlinear refractive index of the sample is a negative value. For eliminating the influence of nonlinear absorption, the Z-scan curve with an aperture is treated with the Z-scan curve without the aperture. The nonlinear refractive index of this type of azo polymer film is determinated as -5.5×10 -6 cm2/W according to the experimental data of Z-scan curve. The value of azo polymer film is larger 8～9 scales than the common inorganic materials. This material will have a very good prospect in the application of optical storage. The reason of large nonlinear refractive index in this material is discussed. Through pilot study, under the irradiation of linear polarized beam, azo molecules became anisotropic. These molecules just like liquid crystal are relevant each other well, response of molecular orientation to outside irritation is a collective effect, therefore they have have very large nonlinear response.