The invention discloses a method for preparing a low-pour-point lubricant additive from low-priced cotton seed oil. The method comprises the following steps: mixing cotton seed oil, methanol and potassium hydroxide so as to prepare fatty acid methyl esters, uniformly mixing the fatty acid methyl esters, formic acid and petroleum ethers at room temperature, stirring and heating the obtained mixture, dripping a mixture of hydrogen peroxide and phosphoric acid into the obtained mixture, standing the obtained product to carry out delamination, separating a water layer, washing an oil layer by using water until the oil layer is neutral, and carrying out reduced pressure distillation to obtain colorless transparent oily epoxidized fatty acid methyl esters, wherein the epoxy value is greater than 4-7, the iodine value is less than 2-0, the acid value is less than 1, and the yield is 90-94%; adding calcium carbonate into a residual formic acid solution after reaction so as to obtain calcium formate which can be recycled; and carrying out esterification reaction on the epoxidized fatty acid methyl esters so as to obtain epoxidized fatty acid methyl ester mixed fatty acid esters. According to the method disclosed by the invention, atalyst application amount is low, the process is clean, the implementation conditions are simple, and the mass production can be implemented conveniently.