The invention discloses a method for machining a separated type outer ring of a small and medium-sized thin-wall crossed cylindrical roller turntable bearing. According to the method, the outer ring of the bearing is machined integrally and is lathed to form two half bodies; and the method comprises the following steps of: a, lathing, namely lathing the outer ring integrally, calculating the allowance of an end face and a roller path according to the size of the outer ring, connecting the two half outer rings partially, lathing the middles of the insides of the two half outer rings to form cutting grooves, and leaving cut-off guiding openings on outer diameters; b, drilling and performing heat treatment, namely drilling the outer ring integrally before performing the heat treatment, and performing the heat treatment integrally after drilling; c, grinding, namely in the processes of initial grinding, performing initial grinding on the outer ring integrally without cutting the outer ring off; before final grinding, performing hard lathing to cut the outer ring off along the cut-off guiding openings on the outer diameters, and marking assorted marks; and polishing a cut-off surface, and connecting the upper half outer ring and the lower half outer ring by using a connecting hole to form an integral outer ring, and performing final grinding. By the method, the problems of high deformation possibility during machining and difficulty in guaranteeing concentricity after assembly are solved.