There are two challenges in the detection of micro capacitance: firstly, the precision of measuring circuit is low, which couldn’t recognize weak AC signal; secondly, the signal processing circuit couldn’t separate the weak signal from huge noise flow. A measuring circuit based on the transimpedance amplifier is introduced in the paper, whose accuracy is 1fF/ms. The phase-sensitive demodulating circuit based on AD835 is applied in the circuit, which can part the useful signal from noise flow combined with a low-pass filter. The methods mentioned in this paper overcome the difficulties and accomplish the extraction of weak signal.