The invention discloses a method for preparing R-mandelic acid, and belongs to the technical field of preparation of mandelic acid. Commercial racemic mandelonitrile, immobilized nitrilase and commercial macroporous strongly-basic anion resin are used as raw materials and catalyzed to generate the R-mandelic acid; and the R-mandelic acid is obtained by elution, concentration, drying and crystallization. The optical purity of the R-mandelic acid product prepared by the method reaches 98 percent, the R-mandelic acid can be widely applied to synthesis of multiple medicinal intermediates, the yield of the R-mandelic acid reaches over 70 percent, and the utilization rate of the raw materials is high; and the production equipment is conventional, the reaction conditions are mild, material resources are fully utilized, 'three wastes' are not discharged, the production cost is low, and the method is favorable for environmental protection and is an environment-friendly method. The product prepared by the method can be widely applied in the industries of medicine, hygiene, dyes, health care and the like.