The neodymium –based (Nd-based) conversion coating was obtained on the surface of AZ31 magnesium alloy. The formation process of Nd-based conversion coating was examined by weighting experiment and open cruit potential (OCP) test. The coating morphology, micro-structure and composition were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy(XPS) and X-ray diffraction (XRD)，respectively. The variation of morphology between bare sample and coated sample immersed in 3.5 wt.% NaCl aqueous solution was investigated, the results showed that the Nd-based conversion coating can effectively reduce the corrosion speed in 3.5 wt.% NaCl aqueous solution. The composition and corrosion resistance of post-treatment on Nd-based conversion coating were measured by XRD and electrochemical experiments. The post-treatment can perfectly improve the corrosion resistance of the Nd-based conversion coating.