The invention discloses a method for preparing a fatty hydroximic acid collecting agent, which is characterized by comprising the following steps of: stirring alkyl hydroximic acid having more than or equal to 5 to 9 carbon atoms, fatty acid having 5 to 22 carbon atoms and mixed solvent oil having 4 to 30 carbon atoms for reaction at room temperature and then separating an oil phase from a water phase, wherein the oil phase is the fatty hydroximic acid collecting agent. The collecting agent of the invention has good selectivity, high collectivity, proper foaming characteristic and easy dissolution in water. Rare earth metal concentrate is obtained by the following steps of: grinding crude rare earth metal oxide ore; adding sodium hydroxide, water glass, sodium fluorosilicate and sodium sulphide into the grinded ore; adding the fatty hydroximic acid collecting agent of the invention into the mixture; then adding modified starch into the mixture; and finally performing flotation. The collecting agent of the invention is a flotation collecting agent for the rare earth metal oxide which is suitable for industrial production and has simple process and high separation efficiency.