The miniaturization of power equipment has raised higher requirements for the insulation and mechanical properties of epoxy resin. The incorporation of a molecular-scale organic-inorganic network within epoxy is a promising approach to simultaneously enhance the electrical insulation and mechanical strength and toughness. In this work, using inorganic calcium phosphate and organic polyetheramine as the precursors, leveraging the chelating effect of amino groups with Ca2+, organic-inorganic hybrid molecules are synthesized and cross-linked uniformly with epoxy through a facile two-step strategy. This strategy enables the incorporation of ultrafine calcium phosphate units into the epoxy crosslinking network, which effectively addresses the inherent issues of inorganic phase inferior dispersibility and weak binding effect with organic matrix associated with traditional fillers. Owing to the quantum size effect of inorganic units and organic-inorganic bonding effect, the epoxy composite efficiently achieves unprecedented comprehensive performance including electrical breakdown strength (116.7 kV/mm) and flexural strength (136.9 MPa) with merely 0.2 wt% inorganic content. This epoxy composite holds promise for advancing the practical application of conceptual equipment.