Quantitatively characterizing intra-meta-atom coupling is essential for understanding and controlling optical chirality in metasurfaces, yet it remains largely qualitative. Here, we propose a robust route to determine coupling strength based on a phase-transition optical activity phenomenon in chiral metasurfaces. By integrating the Born-Kuhn coupled-oscillator model with full-wave simulations, we reveal a sharp threshold behavior in polarization rotation, governed by a threshold coupling constant. This threshold enables precise extraction of the intra-meta-atom coupling strength, establishing a quantitative link between geometry and optical chirality. Multipolar analysis further allows determination of the damping coefficient, leading to accurate evaluation of the coupling parameter. This framework transforms the qualitative description of chiral interactions into a quantitative metric, offering physical insight and practical guidance for designing metasurfaces with enhanced optical activity toward applications in chiral sensing, enantiomer discrimination, and quantum photonics.

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• Metasurfaces
• Optical activity
• Polarization of light
• Chiral symmetry
• Dipole approximation