In order to make the cycloid gear have good meshing performance, according to the actual working scope of the cycloid gear, the cycloid gear profile was divided into working segment and nonworking segment profile, and the topological modification method was used to meet the gap requirements of the working segment and nonworking segment.For the working section,a corner trim was adopted, so that the working section after modification was a conjugate tooth profile, and a certain amount of tooth side clearance was maintained.In the nonworking section, variable equidistant shape was adopted to make the tooth top and root part produce reasonable gap, and reasonable shape profile was obtained. The equation of cycloid gear expansion and repair tooth surface was derived, and the gap of tooth profile satisfying the work requirement could be obtained by adjusting the shape parameters. From the aspects of initial meshing gap, load distribution, and regression analysis, the rationality of the topology modification was verified.