The invention relates to a high-temperature alloy chemical polishing agent and a preparation method thereof, and belongs to the technical field of high-temperature alloy treatment. The polishing agentcomprises the components of 50 g / L to 70 g / L of sulfuric acid, 30 g / L to 70 g / L of phosphoric acid, 5 g / L to 7 g / L of sodium methylene bis-naphthalene sulfonate, 1 g / L to 1.5 g / L of sodium dodecyl sulfate, 3 g / L to 6 g / L of glycerinum, and the balance being water. The preparation method comprises the steps that phosphoric acid and sulfuric acid are mixed and are fully stirred, the temperature isdecreased to the room temperature, and a component A is obtained; sodium methylene bis-naphthalene sulfonate, sodium dodecyl sulfate and glycerinum are added into water, and a component is obtained after stirring; the component A is added into the component B, the components are fully stirred, and the polishing agent is obtained. According to the high-temperature alloy chemical polishing agent andthe preparation method thereof, the polishing agent does not contain nitric acid and hydrochloric acid, less volatile harmful gases are generated during operation, stability is good, environmental pollution is low, and surface quality of a polished high-temperature alloy is high; and in the method, the situations that during mixing, the temperature of the polishing agent is increased as sulfuricacid releases heat, and harmful gases are generated due to decomposition of nitric acid are avoided.