Considering the tooth error of a helical gear is three dimensional, the meshing stiffness analytic method for the helical gear pair after tooth profile modification meeds to be different from that for general spur gears. A calculation method for the meshing line length and position in the threedimensional space of the helical gear was proposed to achieve the rapid calculation, and an analytical calculation model for the meshing stiffness of the helical gear pair considering the axial deformation and tooth profile was put forward according to the principle of force and deformation decomposition combined with the introduction of the coupling relation Between the stiffness and error. Then taking a set of helical gears as an example, the variation of the meshing stiffness of the helical gear pair and the meshing line length in one meshing period was investigated and the influences of different profile modification parameters on the meshing stiffness of helical gear pair were analyzed. The results show that by the model, not only the meshing stiffness of helical gear pair, can be accurately calculated but also the optimal profile modification, can be determined, which can provide a theoretical guidance for the profile modification of helical gears.