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We propose and demonstrate a chiral metasurface with capability for high Q-factor, near-unity (0.999) circular dichroism (CD) via utilization of bound states in the continuum. The CD response can be reversed with the value ranging from −0.7 to 0.82 by increasing structural perturbations to induce chiral magnetic quadrupole modes and the accompanied phase jump. In addition, more than 100-fold enhancement of C/CCPL in the superchiral near field is achieved while maintaining a high Q-factor of 105. The findings hold broad application prospects in chiral sensing, valley electronics, and ultrasensitive chiral spectroscopy detection. [ABSTRACT FROM AUTHOR]