The invention discloses a phosphogypsum-based fabricated exterior wallboard and a preparation method thereof. The exterior wallboard comprises a phosphogypsum component, a sand component, a cement component, a water reducer component, a retarder component, a waterproof agent component, a defoamer component and a fiber component. The phosphogypsum exterior wallboard has an apparent density of below1600kg/m<3>, a breaking strength of 9 MPa or above, a compressive strength of 20 MPa or above, a 24 h soaking water absorption rate of 5% or below and a softening coefficient of above 0.9. The exterior wallboard has the advantages of simplicity in production, no special production device, high strength, low water absorption rate, and meeting of related standards of building materials; and the preparation method has the advantages of simplicity, easiness in implementation, wide source and low price of materials, comprehensive utilization of phosphogypsum, reduction of the weight of buildings,obvious improvement of the quality and the comfort effect of the buildings, and promotion of the comprehensive utilization of the phosphogypsum to exterior walls, and meets the related technical standards of buildings.