This paper proposes a low-power precisely detecting method for electrical-cardiac signal and an efficient signal processing architecture. Traditionally, an impedance channel is designed separately as well as the ECG channel to eliminate the effects of motion-artifact which means more power consumption. The baseline drift and power-line disturbance can also effects signal quality and should be well controlled. In this design, a couple of electrodes and the front-end circuit are reused for both ECG and impedance signal detecting, so that the power-consumption is significantly reduced. Experimental results show that with this front-end architecture, the baseline drift is also well controlled and the power-line effect is reduced. With the proposed digital signal processing method, the location of QRS complexes are precisely detected, which is crucial for medical purpose.