The invention provides a diesel lubricity improver and a preparation method thereof; naphthenic acid with the molecular weight between 400 and 480 and C6-C12 monohydric alcohol or C2-C4 dihydric alcohol are mixed in the ratio of molar number of 1:1.2-2.5, and are reacted in the presence of a catalyst and a water carrying agent at 160 to 190 DEG C for 2-4h to obtain reaction product naphthenate; the naphthenic acid and the naphthenate are mixed according to the mass percentage of 10-30%:90-70%, and heated and stirred at 50-80 DEG C for 1-2H to obtain the diesel lubricity improver; the catalyst is p-toluene sulfonic acid; the water carrying agent is toluene; the dosage of the catalyst is 1.5-2.5% of the total mass of the reaction materials, the dosage of the water carrying agent is 18-25% of the total mass of the reaction materials; the diesel lubricity improver can effectively improve the lubrication performance of diesel, the lubricity-grinding crack diameter of the diesel added with the diesel lubricity improver is less than 460 mum.