The galvanic corrosion behavior of magnesium alloy and Q235 steel system different Sc/Sa(negative / anode area ratio) in simulated solution of tank bottom water in Daxie oil depot was studied by immersion experiment, electrochemical test and micro morphology observation method. The impact of different Sc/Sa on corrosion rate was found out, which curve and expression were fitted. The results show that, the corrosion rate of magnesium alloy anode increases greatly with the increasing of Sc/Sa, but the increasing of corrosion rate for the negative part is not obvious. Based on the galvanic current image, the galvanic current fluctuates and decreases with time. When Sc/Sa is 1.0 or 2.0, the galvanic current decreases obviously, which does not decrease obviously when Sc/Sa is 0.5. The corrosion rate is calculated by galvanic current. It is demonstrated that the galvanic current, as well as the corrosion degree increases linearly with the increase of Sc/Sa.