The invention discloses a high-strength, corrosion-resistant and medium-entropy alloy and a preparation method thereof. The component of the medium-entropy alloy is (CoCrNi)(100-x)Mnx, the atomic ratio of Co to Cr to Ni is (0.5-1):(0.5-1):(0.5-1), the value range of x is 1at%-10at%, , the alloy is formed through mixing, smelting, homogenizing treatment, rolling and heat treatment machining, the grain size of the alloy ranges from 30 mu m to 200 mu m, the manganese-rich phase is separated out of the grain boundary, the tensile strength of the alloy is higher than 1,200 MPa, the ductility is higher than 10%, the corrosion potential ranges from negative 0.43 V to negative 0.26 V, and the medium-entropy alloy is applicable to marine engineering equipment. The preparation process route of the alloy includes vacuum smelting, suction casting, homogenizing annealing, cold rolling and heat treatment. Co, Cr and Ni with the purity being 99% or higher are selected and weighed according to the proportion and are smelted repeatedly in a vacuum smelting furnace, suction casting forming is performed after the component is uniform, multi-pass cold rolling is adopted after annealing treatment, a plate which is 1-2 mm thick is obtained, the tensile strength of the plate is higher than 1,200 MPa, the ductility is higher than 10%, and the corrosion potential ranges from negative 0.43 V to negative0.26 V.