Detailed analysis is carried out upon the holographic modal wavefront sensor (HMWFS) based on the scalar diffraction theory. An analytical expression of intensity distribution of the diffraction field on the detector plane of the HMWFS is presented, which results in the unification with the theory of mode-biased wavefront sensor. For the sake of simplicity in numerical simulation of the HMWFS, an equivalent model of the HMWFS is proposed, in which two tilt conjugate plane waves are chosen to replace the tilt convergent spherical waves as the reference waves in recording holograms, and place a convergent lens in close proximity to the rear surface of holographic element. Then the operation principle of the HMWFS is validaled by numerical simulations employing the equivalent model. The results demonstrates the high sensitivity and fast sensing speed of the HMWFS.