The work aims to improve corrosion resistance of AZ91D magnesium alloy. Protective Ni-Co alloy coating was prepared on AZ91D magnesium alloy by performing electroless plating pretreatment. Morphology, microstructure feature and chemical composition of the coating were analyzed by using environmental scanning electron microscope (ESEM), X-ray diffractometer (XRD) and energy dispersive spectrometer (EDS), respectively. Corrosion resistance of Ni-Co alloy coating for AZ91D magnesium alloy in simulated marine environment (neutral 3.5wt.% NaCl solutions) was analyzed and tested by using potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS). The electrolessly plated Ni-P coating on magnesium alloy was uniformly covered, cauliflower-like grains grew densely, and mass fraction of P element in the Ni-P coating was approximately 5.6wt.%. The Ni-Co alloy coating had uniform coating surface and was in pyramid-shape, formed face-centered cubic (fcc) solid solution, mass fraction of Co in the coating was approximately 31wt.%. Thickness of Ni-P and Ni-Co alloy coating was approximately 11 μm and 19 μm, respectively. In simulated marine environment (neutral 3.5wt.% NaCl solution), corrosion potential (Ecorr) of bare magnesium alloy, Ni-P and Ni-Co alloy coatings pretreated by electroless plating was ?1485 mV, ?372 mV and ?284 mV, respectively; corrosion current density (Jcorr) was 3.4×10?5 A/cm2, 1.8×10?6 A/cm2, 2.9×10?7 A/cm2, respectively; and fitted charge transfer resistance (Rct) was 4.72×103 Ω/cm2, 1.70×104 Ω/cm2 and 2.06×106 Ω/cm2, respectively. The Ni-P coating pretreated by electroless plating can provide better corrosion protection for magnesium alloy while Ni-Co alloy coating provide more significant corrosion resistance for magnesium alloy.