The deposition process of electroless plating of Ni-P on AZ91D magnesium alloy was investigated by metallographic microscope, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Results show that the initial deposition mechanisms of Ni-P coating in different places of the activated surface are different. There is only nickel (Ni) deposition but no P on “massive object” generated after activation, mainly due to the dissolution of the fluorides and the replacement of Mg for Ni. For β phase and the edge of α phase, adjacent Ni2+ would gain a few electrons lost by Mg and be reduced because of electrochemical effect. The Ni nucleus with high catalytic activity provides the catalysis condition for the reduction of P and Ni.