The invention discloses a heat setting device and method of a flat-gap piston ring, relates to the heat setting device and method of the flat-gap piston ring, and aims to solve the deformation problem of a conventional flat-gap piston ring during heat setting. The device comprises two semicircular clamping devices (1), at least two excircle pull rods (2), at least six wedge blocks (3), a pressing plate (4) and at least four end face pull rods (5), wherein the semicircular clamping devices (1) have outer edges and bottoms; the two ends of the semicircular clamping devices (1) have connecting parts; pull rod holes (6) are formed in the connecting parts; two second pull rod holes (9) are formed in each bottom; the end face pull rods (5) penetrate through the second pull rod holes (9) in the bottoms of the semicircular clamping devices (1) to be connected with the semicircular clamping devices (1); and four pull rod holes (6) are uniformly formed in the pressing plate (4), and correspond to the four second pull rod holes (9) of the bottoms when the two semicircular clamping devices (1) are oppositely buckled. The device is applied to the field of manufacturing of piston rings.