The effects of the exchange-coupling between nano-grains on effective anisotropy and coercivity of Nd-Fe-B based nanocomposite permanent magnets were discussed, and the coercivity mechanism of the magnets was also analyzed. The exchange-coupling between hard grains promotes the movement of domain walls and the reversal of magnetic moment at the applied field, which decreases the coercivity of nanocomposite magnets. Therefore, nonmagnetic grain boundary phases with suitable thickness can weaken the exchange-coupling between nano-grains of hard magnetic phase to improve the coercivity of nanocomposite permanent magnets. Besides the nucleation field and self-pining, the pining effect of nonmagnetic precipitates distributed in grains may impede the movement of the domain walls to further increase the coercivity of nanocomposite magnets.