The invention discloses a modified limestone powder, and a preparation method and concrete thereof. The modified limestone powder is prepared from, by mass, 94-97% of a limestone powder, 1% of a physical modifier and 2-5% of a chemical modifier, wherein the chemical modifier is a mixture of oxalic acid, phosphate tailings and a polyferrosilicon flocculant. The limestone powder is subjected to physical and chemical dual modification, the sulfate corrosion problem of limestone powder concrete in the prior art is solved, and the obtained modified limestone powder has the advantages of high compactness, low porosity and excellent sulfate corrosion resistance.