The constrained layer damping plate with the damping layer having periodically distributed square holes was designed. The equivalent elastic tensors of the porous damping layer were calculated by using homogenization method. The finite element model of the constrained layer damping plate considering the homogenization of the porous damping layer was developed and its loss factor was calculated. The size of square cells was optimized to maximize the loss factor of the porous damping composite plate through searching the optimal equivalent shear modulus of the porous damping layer. The numerical example was given to demonstrate the effectiveness of the proposed approach. The results show that the optimization of the square cells can enhance vibration energy dissipation of the porous composite plate and achieve an effective attenuation of amplitudes. In addition, the consumption of the damping material can be reduced significantly. This method demonstrates a potential application in structural vibration control.