The invention discloses a profile modification method for a cycloidal gear, the cycloidal gear and a RV reducer. The method utilizes an ellipse to cut the theoretical conjugate profile of the cycloidal gear in a rolling mode. The center of a needle gear is the center of the ellipse. When the needle gear rolls on a theoretical profile line, the build-out ellipse concentrically rolls with the needlegear, and the angle of rotation of the ellipse during rotation is consistent with the angle of rotation of the needle gear. The profile modification method comprises the steps: when the needle gear is near the tooth root or the tooth top of the cycloidal gear, cutting the profile for modification by using the long axis side of the ellipse; when the needle gear rotates to the working segment of the profile of the cycloidal gear, cutting the profile on the working segment for modification by using the short axis side of the ellipse; and controlling the shape of the modified profile by controlling the shape parameters of the ellipse. The method is simple and flexible, has high controllability, is convenient for subsequent numerical optimization solution, is helpful for a profile modificationdesign, has high application values, changes the traditional profile modification mode, and provides a new idea for the study of profile modification.