Based on the analysis of Electrocardiograph (ECG) signal’s characteristic, a pre-processing circuit contains amplifiers and de-noising filters for ECG monitoring system is designed. In the stage of low-pass filter, we improve the maximally flat (MF) filter and make simulation experiments. It can be proved that, by introducing a pair of??-axis zeros in the transfer function of MF filter, a good performance on both the smoothness in the pass-band and attenuation in the stop-band can be obtained, and the cutoff is much sharper than that of Butterworth filter for the same order. The circuit has been tested on ECG acquiring experiments on human. Both qualitative and quantitative results show that the proposed pre-processing circuit provides good results.