In recent years,some progress has been made in the study of the Nonlinear optics and Nonlinear Optical NLO NLO) properties of Black Phosphorus Quantum Dots (BPQDs),but the size dependNLO NLO properties of BPQDs remain to be studied.In this paper,the NLO behavior of BPQDs with different sizes under 5 nm has been studied.The results show that the NLO absorption coefficient and polarizability of BPQDs with size of 2.3 nm are nearly twice as large as those of BPQDs with size of 4.1 nm,due to the quantum confinement effect and the variation of the photoinduced dipole moment,finite difference time domain method (FDTD) simulations confirm this.This study confirms that nonlinear optics BPQDs is an effective way to enhance their efficiency and shows potential applications in the fields of optics and optoelectronic devices.