The corrosion critical relative humidity and corrosion kinetic properties of Mg-Gd-Y-Zn forged magnesium alloy in humid environment were investigated by scanning electron microscope observation， elemental analysis and corrosion weight gain measurement. The results show that （1） the critical relative humidity for corrosion of Mg-Gd-Y-Zn forged magnesium alloy is 40% to 45%， and the corrosion reaction of magnesium alloy starts to occur when the ambient humidity is ≥40%； （2） the activation energy of corrosion reaction of Mg-Gd-Y-Zn forged magnesium alloy is E_a=0.382 eV， and the accelerating constants of the corrosion of magnesium alloy with different relative humidity are m=3.54702.