Abstract: Based on a common tangent double-circular-arc tooth profile expressed with parametric Heaviside function, the influence of the original tooth profile on the envelope existent domain and feasibility of the conjugated tooth profile are studied with an exact conjugate algorithm using assembly deformation of the flexspline, and a design method is presented to make the double conjugate phenomenon available. The tooth profile orientation on the deformed neutral curve of the flexspline is determined by a rotation angle caused by circumferential displacement on the neutral curve, and a rotational angle of the symmetric line of the tooth profile relative to the radial vector. Then, the movement trace of the tooth profile on the flexspline is plotted, and the tooth profile of circular determined with a series moving tooth profiles on the flexspline is obtained, which verifies the double conjugate phenomenon and the feasibility of the conjugated tooth profile. The illustration example shows that the harmonic drive with double-circular-arc tooth profile can realize continuous conjugate transmission in the whole meshing range by reasonable design parameters of the original tooth profile, and makes more teeth in meshing, which leads to higher transmission capability and stable location accuracy.