The invention provides fluoride-free alkyl carboxylic acid ionic liquid of a structure shown in the general formula (I) (please see the formula in the description). R1-R4 are selected from alkyl groups with carbon atom number of 1 to 18 or benzyl groups respectively and independently, R5 is selected from alkyl groups with carbon atom number of 1-6 or groups shown in the general formula (II) (please see the formula in the description), and n is selected from any integer between 2 and 12. Adopted raw materials are low-toxicity and low-price quaternary ammonium salt and sodium fatty acid, and therefore the fluoride-free alkyl carboxylic acid ionic liquid is low in synthesis cost and toxicity; fluoride elements are not contained, and therefore hydrolysis stability is high, the corrosion to a metal substrate material is avoided, and the liquid is a green and environment-friendly liquid lubricant, can be used as a lubricant for a steel/steel or steel/copper or steel/aluminum friction pair, and has excellent friction reduction and resistant performance.