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The corrosion behavior of Mg–7Gd–3Y–0.4Zr (GW73K) was investigated in as-cast (F), solution-treated (T4) and peak-aged (T6) conditions using immersion tests and electrochemical measurements in NaCl solution (5 wt. ％). Microstructure analyses were carried out on GW73K after different heat treatments by optical microscope (OM), field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM) and X-ray diffraction. It is found that GW73K alloy exhibits high corrosion resistance in T4 than in F and T6 conditions due to the fully dissolution of cathodic coarse (Gd+Y) rich eutectic compound. The corrosion products of GW73K have different morphologies for F, T4 and T6 conditions. The product for in F condition is less uniform and compact than T4 and T6, and it has been founded that GW73K-T6 had two different morphologies owing to the presence of β’. The potentiodynamic polarization curves show that the overpotentials of cathodic hydrogen evolution reaction in T4 and T6 are much higher than in F condition. The anodic curves of GW73K in T4 and T6 had clear current plateaus due to the protective corrosion product film formed on the surfaces of the alloys. The results of potentiodynamic polarization curves also confirm that the heat treatment is beneficial to the improvement of the corrosion resistance for GW73K alloy by transforming the microstructures.