A facile approach has been adopted for coating cross-linked polystyrene (PS) shells on the surface of Fe3O4 magnetic clusters using reflux-precipitation polymerization (RPP). Treating the PS shell with chlorosulfonic acid yields magnetic composite particles with acid functionality. By adjusting the amount and proportion of monomers (styrene and divinylbenzene), the obtained magnetic composite particle solid acid (MPM-5S) exhibits a saturation magnetization value of 18 emu/g, a specific surface area of 243 m2/g and an acid density of 2.113 mmol/g. The MPM-5S magnetic solid acid catalyst was evaluated for esterification of oleic acid with methanol to prepare biodiesel. Under mild conditions, the conversion of oleic acid reached 91% , which was much higher than the catalytic activity of Amberlyst-15 and close to the catalytic activity of concentrated H2SO4. The solid acid catalyst can be recovered by magnetic separation and reused three times maintaining over 95% of its initial catalytic activity. Additionally, the solid acid can be used to catalyze the dehydration of fructose to 5-hydroxymethylfurfural.