Abstract: A method for preparing graphene by reaction with Cl2 based on annealing with assistant metal film is provided, comprising the following steps: applying normal wash to a Si-substrate, then putting the Si-substrate into a reaction chamber of a CVD system and evacuating, rising the temperature to 950° C. -1150° C. gradually, supplying C3H8 and carbonizing the Si-substrate for 3-10 min; rising the temperature to 1150° C.-1350° C. rapidly, supplying C3H8 and SiH4, growing a 3C—SiC hetero-epitaxial film on the carbonized layer, and then reducing the temperature to ambient temperature under the protection of H2 gradually, introducing the grown sample wafer of 3C—SiC into a quartz tube, heating to 700-1100° C., supplying mixed gas of Ar and Cl2, and reacting Cl2 with 3C—SiC to generate a carbon film, applying the sample wafer of carbon film on a metal film, annealing at 900° C.-1100° C.