An optimized structure for a special digital filter was designed. The structure of filter could be simplified and the area of the circuit could be reduced effectively. It was applied to the demodulation of ISO/IEC 14443 in communication between PICC and PCD. Having changed the position of the sub-filter and moved the long delay chain forward, the bit width could be reduced. Finally, according to the results of synthesis, the area of the filter was reduced by 50% compared with that of the traditional filters.