Mg-Bi based alloys, which are ideal materials for the development of new magnesium alloys, are known as “green metals” and are widely used in automotive, aerospace and other fields. To compensate for the lack of corrosion resistance, a new type of low-alloy Mg-1Bi-0.3Ca magnesium alloy was designed by microalloying in this work, and the corrosion behavior of a Mg-Bi based alloy in a 3.5 wt.% NaCl aqueous solution was investigated by adding 0.3 wt.% Ca. Hydrogen evolution and electrochemical experiments reveal that the addition of 0.3 wt.% Ca improves the corrosion resistance of the alloys, and the Nyquist plots of the two samples consist of two loops, one high-frequency capacitive loop and one short low-frequency inductive loop. The Mg-1Bi-0.3Ca alloy has the lowest corrosion rate, with a value of 0.365 mm/y. The XRD and metallographic results show that the addition of 0.3 wt.% Ca does not change the type of second phase in either alloy, which is the Mg3 Bi2 phase, but influences the distribution.