The Shilu iron ore deposit in Changjiang County of Hainan Province is the largest high-grade iron deposit in China. Mesozoic intrusions are widely distributed in and around the ore district, Intense effect of magmatism on the ore deposit yielded two types of reformed ores, i.e., garnet-magnetite ore and pyrite-magnetite-hematite ore. Mineralogy researches on the reformed ores, together with the evidence from sulfur isotope and trace elements of ores show that hematite changed into magnetite during the reformation, with sulfur mainly derived from magma, and iron and trace elements derived from the original ore. Considering the geological characteristics of the ore deposit, it is concluded that the magmatic reformation action occurred later than the enrichment of the Shilu iron deposit, probably belonging to a kind of thermal reformation which resulted in the magnetization of hematite without the enrichment of the ore.