To investigate the deposition mechanism of the outermost electroless nickel plating layer in a three-layer composite coatings on the magnesium alloy surface, the electroless nickel plating layers with six different plating times (3, 5, 10, 30, 60, and 90 min) were selected as research targets. The transformations in microcosmic morphology, element composition and material structure of the samples electrolessly plated at six different plating times were characterized by SEM. EDS and XRD. The changes in corrosion resistance of samples with different electroless plating times were measured by polarization curve. It was concluded that the nickel-phosphorus deposition process on the activation composite coatings was a "cell-like three-dimensional" growth judging from the microscopic morphology of coating surface obtained by SEM. The elemental changes obtained from the EDS were consistent with the growth of nickel cells obtained SEM. The XRD results showed that the diffraction peaks of nickel were not detected on the surface at an electroless plating time of 3 min. Nickel peaks appeared after 5 min of electroless plating, and the nickel peak width broadened and the intensity increased as the electroless plating time increased. The corrosion resistance is greatly improved when the electroless plating is performed for 60 min due to the complete nickel coating without defects as shown in the SEM. It can be concluded that the deposition mechanism of electroless plating on the double-layer active surface is an initial ecological reduction [H] autocatalytic reduction deposition process according to the micromorphology and structure of the samples electrolessly plated at six different plating times. (C) 2019 Elsevier Ltd. All rights reserved.