Abstract: Electrolytic copper foil (ECF) is a fundamental material for lithium-ion batteries and electronics, where mechanical properties are critical. This study combines theoretical and experimental methods to investigate halogenated phenylthiourea (X = F, Cl, Br) as ECF additives. Density Functional Theory (DFT) calculations revealed that the molecular orbital energy gap of these additives initially decreases and then increases from ortho to para substitution, while generally decreasing with reduced halogen electronegativity. Molecular dynamics simulations confirmed their strong adsorption on various copper crystal planes. Guided by theory, experiments identified 3-fluorophenylthiourea as the optimal additive, producing ECF with a tensile strength of 731.71 MPa, surpasses that of the previously reported extremely thin copper foil. A clear trend emerged: mechanical properties improved with increasing halogen electronegativity (F > Cl > Br) and, for a given halogen, meta-substitution was vastly superior to ortho or para positions. Microstructural analysis (SEM, TEM, EBSD) showed that the superior performance from the meta-fluoro additive stems from significant grain refinement and a smooth, flat foil surface.