A dense thick alloy coating with no layered organizational structure and a preparation method thereof are provided. The preparation method includes the following steps: step 1, placing a substrate in a plasma spraying apparatus, controlling a vacuum degree of the plasma spraying apparatus to be 0.2 mbar-0.5 mbar, controlling a length of a plasma flame of the plasma spraying apparatus to be 1000 mm-1200 mm, and controlling a diameter of the plasma flame to be 200 mm-300 mm; step 2, preheating, by using a plasma spray gun, the substrate to a temperature of 700° C.-1000° C. to obtain a preheated substrate; step 3, plasma spraying an alloy powder on the preheated substrate under a specific plasma spraying condition to obtain a sprayed substrate; and step 4, cooling the sprayed substrate after the plasma spraying, to thereby obtain the dense thick alloy coating without layered organizational structure.