The invention relates to hydroxyl acrylic emulsion of a core-shell structure and a preparing method thereof. Monomers forming an emulsion particle shell layer are mixed monomers composed of 5-15% of hydrophilic monomers, 5-50% of functional hydroxy monomers, 1-10% of associated thickening monomers and 20-60% of vinyl monomers. The hydrophilic monomers are acrylic acid or methacrylic acid; the functional hydroxy monomers are epsilon-caprolactone modified vinyl monomers containing primary hydroxyl groups, the structural formula of the functional hydroxy monomers is CH2=CRCOOCH2CH2O[CO(CH2)5O]nH, R represents H or CH3, and n=1-6 or n=10; the associated thickening monomers are docosyl alcohol polyoxyethylene ether methacrylate. The emulsion has a self-thickening effect. By adjusting the pH value of the emulsion, the viscosity of the emulsion can be improved substantially, and the emulsion has good anti-shearing performance. A coating of water-based paint obtained by compounding the emulsion and a curing agent has excellent water resistance.