Using the part exchange-coupling model, the exchange-coupling interaction between magnetically soft and hard grains in Nd2Fe14B/a - Fe nanocomposite and its effect on the effective anisotropy of the material have been investigated. By testing six different functions, series curves of effective anisotropy constant versus the mean grain size were obtained. The calculation results show that the effective anisotropy of the composite increases slightly with decreasing grain size, reaches the maximum at about 23nm, and then decreases abruptly. The calculated curves resemble the experimental results of the coercivity versus grain size in nanocomposite.