A method for measuring metal ion concentration includes the following steps of: providing a measuring solution having a corrosion inhibitor; adding a specific quantity of metal ion solution into the measuring solution; providing a potential scan device (1); measuring a cyclic voltammetry curve of the measuring solution using the potential scan device at a constant scan rate in a specific potential range, the cyclic voltammetry curve has a peak current; obtaining a linear equation, which indicates a linear relationship of peak current versus a concentration of corrosion inhibitor in standard corrosion inhibitor solution in the specific potential range, the peak current is a peak current of cyclic voltammetry curve of the specific quantity of metal ion solution metal ion in the standard corrosion inhibitor solution in the specific potential range; and determining a concentration of the corrosion inhibitor of the measuring solution by computing the peak current of the cyclic voltammetry curve of the measuring solution in linear equation.