Combined with water resistance of magnesium oxychloride cement concrete, the corrosion and protection for steel bar in magnesium oxychloride cement concrete were experimentally investigated, and the influencing factors included: the type of steel bar, thickness of the concrete protective layer and the corrosion duration. The two types of steel bar were bare steel bar and MAGNI coated steel bar. The two types of thickness of the concrete protective layer were 25 mm and 50 mm. The six types of the corrosion duration were 60 d, 120 d, 180 d, 240 d, 300 d and 360 d. 2/3 height of test block was prolonged soaked in water. By SEM and EDS, microstructure and chemical elements of steel bar with corrosion were analyzed to study corrosion mechanism. Result showed that by softening coefficient, the softening coefficient of magnesium oxychloride cement concrete was from 0.78 to 0.87, and the magnesium chloride cement concrete designed by this experiment could be used as a dry area, damp lighter area or secondary structure. By polarization curves and its electrochemical parameters, corrosion rate of bare steel bar was from 40 to 80 times as large as corrosion rate of MAGNI coated steel bar.