A generalized supercell model is established to predict band structures of phononic crystals (PnCs) microplate with or without defects. Microstructure effect in elastic flexural waves is described by the modified couple stress theory. The plane wave expansion (PWE) method and the supercell technique are applied to solve the wave equations. The generalized supercell model contains P×Q square inclusions that can be adjusted according to different situations. The current model can be used to solve various defect state problems of multi-phase materials (arbitrary) composite PnCs microplates. Single point defect and multiple defects cases are calculated. The results demonstrate that the current model can well predict defect states of the PnCs microplates whether there is coupling between defects and supercell or not. This new model can provide help for the design and application of PnCs with multiple defects at the microscale in different fields.