The invention discloses a lubricant for heavy-load machining of metals and a preparation method thereof. The lubricant comprises mineral oil, component A, component B, component C, component D, component E and an antirust agent, and the sum weight percentage of the above-mentioned raw materials is 100%. The preparation method is as follows: adding mineral oil in a container capable of stirring and heating, allowing the temperature of the container to rise to 75 to 80 DEG C, adding component A and component B, allowing the temperature of the container to maintain 75 to 80 DEG C, allowing the mixture in the container to react for 30 min, adding component C and the antirust agent, carrying out insulation and an reaction for 30 min, adding component D, carrying out insulation and an reaction for 30 min, cooling the container to a temperature of 30 DEG C, adding component D, and carrying out filtering and packaging so as to obtain a finished product. The lubricant for heavy-load machining of metals provided in the invention costs little, is nonflammable and has excellent performance. The preparation method for the lubricant in the invention has the characteristics of low cost, simple operation and easiness in large scale popularization and application.