Provided is a method for room-pressure and low-temperature preparation of graphene, comprising: heat treating a compound of graphite oxide and sulfuric acid in either room pressure or negative pressure at a temperature between 50 and 400° C., thus converting graphite oxide into graphene. Also provided is a method for low-temperature preparation of a graphene composite material. The acquired graphene and graphene composite material are applicable in optical materials, electrically-conductive materials, sensor materials, catalytic materials, battery materials, and supercapacitor materials.