As a thermal energy storage medium (TESM) at moderate-high temperature of concentrating solar power (CSP), the molten NaCl-MgCl2 has sharp corrosive action on a metal container. The influence of MgCl2 content on a metal corrosion behavior is not clear. The corrosion behavior of a Fe based alloy (GH1140) in molten NaCl-MgCl2 (at 1123 K) with different content of MgCl2 (wt% : 0.0, 48.9, 61.0, 93.6) were studied by the submergence corrosion experience. Results show that: after corrosion for 5 h, there is a negative correlation between the weight loss of samples and the content of MgCl2. In the molten salt without MgCl2, the sample was corroded according to the mechanism of "dissolution as anode-oxidizing-peeling off of oxide film". After adding a little MgCl2 in the molten salt, an unprotected MgO shell was formed on the sample surface. The samples weight decreased and obeyed a line law. The elements of Fe and Cr deplete in the corrosion layer. Corrosion mechanism is "dissolution as anode-oxidizing-reduction-chlorination". When the content of MgCl2 is very high in the molten salt, partial pressure of Cl-2 increases. The corrosion mechanism is similar to what in the molten salt with a little MgCl2, but the chlorination reaction is more severe.