The third-order nonlinear refractive response in a kind of thin phthalocyanine film has been investigated using two types of top-hat Z-scan techniques with a picoseconds pulses laser at wavelength of 532nm. The nonlinear phase shift of this sample is so week because of the very small thickness of the films, and the normal top-hat Z-scan technique is often difficult to measure the results. To research the nonlinear response of this thin film, we also has been measured it using the modified top-hat Z-scan technique under the experimental conditions of the same. The curve of the modified top-hat Z-scan for the nonlinear refraction is only a single peak and the result shows this kind of film has obvious response of nonlinear refraction with 19 picoseconds pulses. Furthermore, we have been carried on the theoretical analysis and the numerical simulation with the result of the experiment, and the theoretical simulation fit well with experimental results, the value of the nonlinear refraction index is obtained to be <i>n</i><sub>2</sub> = 6.25×10<sup>−15</sup> <i>m</i><sup>2</sup> /<i>W</i> . By comparing the measured curve of the two, the results indicate that the sensitivity of the modified top-hat Z-scan measurement is greatly enhanced.