Both theoretical analysis and practical application prove the existence of the deflection of the flexspline teeth. The change of meshing trajectory and backlash of circularspline and flexspline on the different section shows that the tooth deflection makes the axial part of flexspline teeth near the bottom of the tube easy to engage at the front part of meshing time,then meshing is easy to appear on the axial part of flexspline teeth near the mouth of the tube. The most meshing of the section on the conjugate meshing area is interfered. The meshing interference caused by tooth deflection occurs earlier in the non working side of the tooth. The meshing interference makes the force of the flexspline increase,and produces periodic bending deformation of the flexspline,which may make fatigue crack. The different wave generators have a certain influence on backlash fluctuations,but not very seriously.