The AgInSbTe phase change thin films are very important as optical recording and the super-resolution mask materials in high density optical information storage. In this work, the effective nonlinear absorption coefficients of amorphous and crystalline AgInSbTe thin films were measured by the open-mode Z-scan method and no evidence of nonlinear refraction was found in the closed-mode Z-scan measurement. The effective nonlinear absorption coefficient βeff of amorphous AgInSbTe thin films is 7.53×10−3 m/W and the effective photon-absorption number n is 1.722; βeff of crystalline AgInSbTe thin films is 3.5×10−2 m/W, which is of an order lager than that of amorphous state, and the n value is 1.7011. The giant nonlinearity of AgInSbTe results from the free carrier absorption in the nanosecond time scale and this characteristic should be responsible for the mechanism of optical recording as well as the readout of super-resolution disk.