The coercivity mechanism of HDDR anisotropic Nd Fe B magnet is investigated. The measurements of magnetization curve and different susceptibilities as well as the observations of microstructure are performed. It is concluded that the magnetic hardening of the magnet is due to the pinning of domain walls at grain boundaries and the Nd rich boundary phase that distributes homogeneously around some colonies composed of fine Nd2Fe14B grains. The coercive field is mainly determined by the pinning of domain walls by the Nd rich boundary phase. The conclusions mentioned above are in good agreement with the theoretical calculation of the intrinsic coercivity of the magnet. Furthermore, experimental verification is carried out and the processes for improving the coercivity of the magnets are proposed.