The invention relates to a B-site Mn and Cu codoped high remanent polarization BiFeO3 film and a preparation method, the method comprises the following steps: dissolving bismuth nitrate, ferric nitrate, manganese acetate and cupric nitrate according to mol ratio of 1.05: [(0.92-0.98)-x]: (0.02-0.08):x in a mixed liquor of ethylene glycol monomethyl ether and acetic anhydride, then uniformly stirring to obtain a BiFeO3 precursor; wherein total metal ion concentration of the BiFeO3 precursor is 0.1-0.5mol / L, X is 0.01-0.03; performing spin coating of the BiFeO3 precursor on a FTO / glass substrate to prepare a wet membrane, baking the wet membrane to obtain a dry membrane, then annealing at 550 DEG C to obtain the crystalline state BiFeO3 film; cooling the crystalline state BiFeO3 film, and repeatedly making the crystalline state BiFeO3 film to reach a required thickness to obtain the B-site Mn and Cu codoped high remanent polarization BiFeO3 film. According to the invention, a sol gel technology is employed, the equipment requirement is simple, the film is prepared on large surface and surfaces with irregular shapes, the chemical component is accurate and controllable, and the regulation and control to its crystal structure can be carried out by codoping thereby the ferroelectric performance of the film is greatly increased.