Abstract: A mirror milling device for machining an arched annular thin-walled workpiece includes an external vertical gantry milling head assembly, an internal support and measurement head assembly, a clamping fixture, and a worktable. The machining method thereof includes: dividing a parallel curve of a theoretical surface and a parallel curve of a reverse surface, performing triangular meshing on a division point of the theoretical surface, using indexes of three points as indexes of a triangle, calculating area coordinates, in triangular meshes, of position points on a machining path, causing points with corresponding indexes on the reverse surface to form a triangle, reversely solving coordinate points by using the area coordinates, and compensating a machining path on a spherical surface from a designed curved surface to an actual curved surface obtained by means of reversing.