The invention belongs to the technical field of spaceflight servo mechanisms, and particularly discloses a harmonic reducer. The harmonic reducer comprises a shell body, wherein a circular spline is fixedly connected with the shell body and is connected with one end of a flexible gear arranged inside the shell body in a meshing mode, and a wave generator assembly is arranged in the end, meshed with the circular spline, of the flexible gear; a shaft installation hole is formed in the bottom of the shell body, an output shaft is arranged in the hole, one end of the output shaft is fixedly connected with the flexible gear, and a supporting bearing is installed on the output shaft in a sleeved mode; an end cover is arranged outside the supporting bearing, an installation hole is formed in the end cover, the end cover is fixedly installed on the shell body and seals the shaft installation hole in the bottom of the shell body, and the output shaft stretches out of the shell body after penetrating through the hole in the end cover. According to the harmonic reducer, due to the design of gear teeth of the flexible gear and the circular spline, stress distribution of tooth roots of the flexible gear and meshing quality of transmission are improved, transmission bearing capacity and torsional rigidity are improved, the axial size of the flexible gear is allowed to be small, and then the quality and size of a product are reduced and the application range of the harmonic reducer is greatly widened.