Abstract: Both the optical limiting mechanism of suspending liquid of multi-walled carbon nanotubes for 532nm laser and the impact of samples thickness on limiting mechanism were investigated by Z-scan test, nonlinear absorption test and nonlinear scattering test. The results show that nonlinear absorption is the main reason in optical limit for lower input energy. Nonlinear absorption of carbon nanotubes, nonlinear scattering of plasma generated by ionization and air bubble generated by solvent boiloff result in a strong limiting factor of carbon nanotubes for high input energy. With the increase of sample thickness, the limiting factor of carbon nanotubes increases, but its nonlinear scattering is reduced when the concentration of suspending liquid is fixed.