The invention discloses a preparation method of modified graft polyvinyl fluoride and the prepared modified graft polyvinyl fluoride. The method includes the steps of: adding a free radical initiator and olefin radical monomers in a polyvinyl fluoride resin solution at the temperature of 65 to 95 DEG C, mixing uniformly, stirring, and reacting at the temperature of 75-115 DEG C for 1-5 hr; the amount of polyvinyl fluoride resin is 100 parts by weight; the amount of free radical initiator is 0.01-15 parts by weight; and the amount of olefin monomer is 0.01-15 parts by weight. The modified graft polyvinyl fluoride resin prepared by the method has better resin solution twist and tensile properties than the polyvinyl fluoride resin obtained by the prior art, and the invention has the advantages of simple preparation process, easily controlled conditions and strong applicability.