The third-order nonlinearities of PbS nano-belts have been investigated using the Z-scan technique with nanosecond laser pulses at 532 nm. The effect of the PbS semiconductor nano-belt size on the third-order optical nonlinearities and optical limiting properties was studied. We found that the nonlinear absorption and nonlinear refraction have not strict dependence on the nano-belt size under our investigated condition, but their optical limiting behavior is different. The optical limiting mechanism of materials is discussed.