CsPbBr₃ nanoplatelets (NPLs) with different thickness (3-5 monolayers) were synthesized by ligand-assisted reprecipitation at room temperature to study the nonlinear optical absorption properties in the near-infrared-II window. The nonlinear three-photon absorption properties of CsPbBr₃ NPLs were investigated by Z-scan technique using a laser with central wavelength of 1030 nm, pulse width of 6 ps and repetition of 25 kHz. The experimental results show that the nonlinear three-photon absorption cross section of CsPbBr₃ NPLs is significantly increased due to the enhancement of quantum confinement effect with decreasing the thickness of NPLs. The three-photon absorption cross section of the 3 monolayers NPLs is 4.1 × 10^-71 cm⁶s²photon^-2. This study shows that CsPbBr₃ NPLs have excellent nonlinear multiphoton absorption properties in the near-infrared-II window, and can be applied in the field of multiphoton fluorescence imaging.