In harmonic gear drive,backlash has a significant effect on transmission performance. Based on the exact conjugate method of harmonic drive,the changes of conjugate zone,circular spline conjugate tooth profile and backlash of harmonic drive with the variation of flexspline tooth profile parameters are studied. The results show that flexspline profile parameters play a decisive role in conjugate section,circular spline conjugate tooth profile and backlash. Reducing the circular arc radius of convex tooth profile is helpful to increase the meshing conjugate zone,but not conducive to reducing the backlash. The circular arc radius of concave tooth profile has no obvious effect on the conjugate zone,but has a significant effect on the backlash. Reducing the common tangent angle is helps to increase the conjugate zone,reduce the backlash and improve the backlash distribution. The circular arc radius of convex and concave tooth profile of flexspline have obvious influence on the circular spline tooth profile parameters,but the common tangent angle have little effect on it. A reasonable choice of flexspline tooth profile parameters may bring a bigger conjugate zone and more well-distributed backlash,the carrying capacity and transmission performance can then be improved.