The invention discloses high-activity mpg-C3N4/RE-BiVO4 heterojunction photocatalyst and a preparation method and application thereof. First, mpg-C3N4 powder is prepared and dispersed into ethyl alcohol; then a bismuth nitrate pentahydrate solution and an ammonium metavanadate solution are mixed, the pH value of the mixture is adjusted, rare earth nitrate is added, and then precursor RE-BiVO4 turbid liquid is obtained; then, an mpg-C3N4 alcoholic solution is added into the precursor RE-BiVO4 turbid liquid to obtain reaction precursor liquid, and finally a microwave hydrothermal method is adopted for preparing the mpg-C3N4/RE-BiVO4 heterojunction photocatalyst. The catalyst is of a heterojunction structure formed by mesporous mpg-C3N4 and rod-shaped tetragonal zircon phase RE-BiVO4, high photocatalytic activity is achieved under irradiation of simulated sunlight, the photocatalytic efficiency of pure-phase BiVO4 is greatly improved, and good application prospects in the aspect of treating environmental pollutants and others are achieved.