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To date, second-harmonic generation (SHG) at nanoscale has concentrated on employing high-refractive-index nanostructures, owing to the strong field confinement at deep subwavelength scales based on optically resonant effects. However, nanostructures with lower index contrast between the structures and the surroundings generally exhibit weaker resonant effects and lower field confinement. To address this issue, by harnessing the large nonlinearity of , we propose an approach to employ guided resonances and bound states in the continuum (BICs) with a metasurface consisting of a disk array sitting on a thin film. Such a system can transform the guided modes supported by a thin film into high-quality guided resonances which can be excited directly under plane-wave illumination. Importantly, we further demonstrate strong field confinement inside a thin film with a tailorable …