Based on the great difference in dielectric constant between water and building materials,a capacitivesensor for the detection of water pipe in walls is proposed.In the investigation,first,a detection model of capaci-tive sensor is established on the basis of the Laplacian equation in spatial static electric field.Then,the model issolved via the relaxation iteration,and the relationship between the output capacitor of the sensor and the position ofthe detected water pipe is acquired.Moreover,the measuring circuit of the capacitive sensor is designed on the ba-sis of the pulse width modulation,with its exciting voltage being adjusted by a fuzzy PID controller unit.Thus,themeasuring circuit can evolve into the working condition as soon as possible.Finally,a center detection algorithm ofwater pipe is designed for the proposed sensor unit,and the corresponding measurement is carried out.The resultsshow that the PID controller helps to adjust the exciting voltage in the measuring circuit rapidly and accurately,which makes the sensor to be effective in different working conditions; and that the proposed system can detect thewater pipe in a wall with the thickness of 120mm and locate the center axis of water pipe in a wall with the thick-ness of 70mm.