How to achieve high-resolution ADCs from low-resolutionones by filtering quantization noise is the key problem of signal processing researchers. Considering IF bandpass signals in communication, a method is proposed to reduce the quantization noise and improve the resolution of A/D conversion by adding a post-bandpass filter. Then the method is analyzed and tested by numerical simulations with a common IF bandpass signal as an example. Results show that the quantization noise is observably reduced,through bandpass filtering for quantized signals. Therefore, the A/D conversion performance are improved obviously. In addition, in the aspect of theory, this paper revises certain traditional views on the statistic model of quantization noise and loosens the assumption condition that the quantization noise is a white one.