The invention discloses a g-C3N4/Al2O3/ZnO ternary heterojunction material with visible light catalytic activity and a preparation method thereof. The g-C3N4/Al2O3/ZnO ternary heterojunction material comprises, by weight 50-70% of g-C3N4, 45-20% of Al2O3 and 5-20% of ZnO. The preparation method of the g-C3N4/Al2O3/ZnO ternary heterojunction material comprises: dissolving suitable aluminum nitrate in distilled water, adding prepared g-C3N4 powder, stirring well, dropwise adding NaOH solution until pH is 8-9, stirring, dissolving in zinc nitrate, dropwise adding NaOH solution to maintain pH to 8-9, continuing to stir, filtering by suction, drying, and grinding to obtain a sample precursor, and calcining at 400 DEG C to obtain the g-C3N4/Al2O3/ZnO ternary heterojunction material. The preparation method is simple, the raw materials are low in rice, complex equipment is not required, the process is free of pollution, and the material is suitable for industrial batch production.