The invention provides a method for preparing a platinum black/ platinum ruthenium black nano electro-catalyst by using a sacrificial magnesia carrier. The method is used for preparing the platinum black/ platinum ruthenium black nano electro-catalyst based on a metallic carbonyl cluster path, and comprises the steps of synthesis of a catalyst precursor, namely a metallic carbonyl cluster, injection of magnesia, heat treatment of an intermediate of the catalyst, dissolution of the magnesia and post treatment of the catalyst. The method specifically comprises the following steps of: introducing CO under an alkali condition to react with the precursor of the electro-catalyst to obtain the metallic carbonyl cluster, adding the magnesia in a protective atmosphere, stirring, and transferring the solvent at the temperature of between 30 and 120 DEG C in the protective atmosphere; performing heat treatment; and adding an acid solution to dissolve the magnesia, heating, performing post treatment, and thus obtaining the platinum black/ platinum ruthenium black nano electro-catalyst with the particle diameter of 2 to 20 nanometers and narrow particle size distribution. The electro-catalyst is suitable for a cathode and an anode of a proton-exchange membrane fuel cell.