In order to truly investigate the backlash distribution between tooth profile of flexspline (FS) and circular spline (CS), a finite element model (FEM) of harmonic drive (HD) based on solid element is established by APDL, and backlashes are calculated with the minimum circumferential distance between engaged tooth profiles along tooth height direction. Taking involute tooth profile parameters and structural parameters of FS and CS into consideration, the FEM of planar tooth ring with involute teeth under action of four-roller wave generator is calculated with contact analysis, and assembly status is obtained. By adjusting the radial position of the roller until the maximum radial displacement on the major axis of the deformed FS reaches the specified value. Backlashes are calculated for all engaged tooth profiles and the backlash distribution curve is obtained. The research indicates that the location of backlash alternately occurs between the addendum of FS and the addendum of CS, and their backlash curves go across twice in the engagement interval. The validity of the backlash computation based on the FEM is higher by the contrastive analysis, which provides a reliable way of backlash computation.