Based on modified multi-field coupling equations, the magnetoelectric (ME) response of a new kind of Ni47.4Mn32.1Ga20.5/PZT multiferroic composites are calculated by considering the mechanical failure of the brittle PZT substance. It is theoretically revealed that the Ni47.4Mn32.1Ga20.5/PZT bilayer composites could produce an ideal giant ME (GME) response up to 120 V/cm Oe, much larger than the best reports up to now. However, the real ME response will be strongly limited by the mechanical strength of the brittle PZT. Reducing the PZT layer and using a mechanically stronger PZT material have been suggested to enhance the ME response.