The present disclosure relates to a charging system for an energy storage device of hybrid construction machinery, and more particularly, to a charging system for an energy storage device of hybrid construction machinery which is capable of computing an amount of regenerable energy predicted according to an operational situation of an actuator, calculating a target charging rate of the energy storage device by reflecting the computed amount of the regenerable energy, and finally, computing an amount of power generated by an engine auxiliary motor in order to compensate for a difference between a target voltage and an actual voltage of the energy storage device, in the case of charging the energy storage device of the electric hybrid construction machinery.