Taking nanocrystalline Nd2Fe14B as a typical sample, based on Herzer′s random anisotropy theory and the cubic grain model, the partial exchange-coupling interaction model was established and the dependence of effective anisotropy constant Keff on grain size was investigated. Calculation results reveal that the exchange-coupling interaction enhances and the effective anisotropy of material Keff decreases with the reduction of grain size. The variation of Keff is basically the same as that of coercivity. The decrease of effective anisotropy is the main reason of the reduction of coercivity for nanocrystalline Nd2Fe14B permanent magnetic material.