The corrosion behaviors and mechanism of the as-rolled Mg-xZn-0.5Er (x=0.5, 2.0, 3.0 and 4.0, in wt.%) alloys were investigated. The potential difference between the second phase and matrix was determined by the type and size of the second phases. The dominated nano-scale W-phase on matrix in the Mg-0.5Zn-0.5Er alloy induced a sharp local pitting corrosion. However, as the volume fraction of the coarse W-phase or I-phase increased, the corrosion reaction was inclined to happen around the coarse W-phase or I-phase preferentially, leading to an obvious uniform corrosion and great acceleration of corrosion rate. The long-term immersion (14 d) results indicated that the corrosion resistance decreasing sequence was Mg-0.5Zn-0.5Er > Mg-2.0Zn-0.5Er > Mg-3.0Zn-0.5Er > Mg-4.0Zn-0.5Er.