The invention relates to modified polyvinyl alcohol, which has the following structure formula defined in the specification, wherein x, y and z are integers, x is more than 1 and is less than or equal to 500, y is more than 1 and is less than or equal to 2000, z is more than 1 and is less than or equal to 200, y is more than x, y is more than z, R1 is an oxygen atom or -CO- or -(CH-OH)-, and R2 is alkyl, alkoxy or hydroxyl-terminated alkyl. According to the present invention, the monomer copolymerization consumption is less, the reaction rate is moderate, the conversion rate of the prepared vinyl acetate-branched vinyl copolymer is 40-80%, the alcoholysis degree of the alcoholysis product is 98-99.8 mol%, the crystallinity is less than 20%, the melting point is 160-195 DEG C, the viscosity is 3.0-25.0 mPa.S (20 DEG C, 4% aqueous solution), and the obtained product can be subjected to melting processing and has good processability.