A Z-scan system is employed to investigate optical nonlinearity of Bi20Sb80 thin films under low laser intensity. Optical constants and ellipsometric parameters are measured by spectroscopic ellipsometry. Experimental results indicate that Sb80Bi20 films show giant nonlinear saturated absorption. The nonlinear absorption coefficient is about -0.018 m/W. However, there is no appreciable nonlinear refraction effect in measurement. The giant nonlinear saturated absorption is dominant response for super-resolution effect. The calculation result indicates that the squeezed half width of Gaussian spot with about 10% can be achieved in Sb80Bi20 films with thickness of 35 nm. The Sb80Bi20 nanofilms are shown to be very promising for super-resolution applications.