The electroplating behavior of nanocrystalline CoNiFe soft magnetic thin film with high saturation magnetic flux density (Bs＞2.1 T) and low coercivity (Hc) was investigated using cyclic voltammetry and chronoamperometry methods in conjunction with the scanning electron microscopy (SEM/EDX). The results show that, under the experimental conditions, the co-deposition of CoNiFe film behaves anomalously due to the atomic radii of iron series elements following the order of rFe＞rCo ＞ rNi. In the case of lower electroplating current density, the co-deposition of CoNiFe film follows a 3-D progressive nucleation/growth mechanism,while in the case of higher electroplating current density, which follows a 3-D instantaneous nucleation/growth mechanism.Meanwhile, the change of nucleation mechanism of CoNiFe film with electroplating current density was interpreted theoretically in the light of quantum chemistry.