The common failures and major monitoring methods of pump were analyzed. Electrostatic sensor technology was introduced into the hydraulics on line monitoring system. The mechanics of oil on line monitoring for pump and the factors that affected the charge quantity were studied. A set of on-line monitoring system used for hydraulics system was designed. The relationship between the measuring voltage of the electrostatic sensor and the concentration, size and material of the wear debris was researched. The monitoring results of pump’s signs of wear were obtained by using the electrostatic sensor to monitor the charged particles of wear debris on-line in the hydraulics oil. The validity and practicability of the electrostatic method used in on line monitoring for pump were proved.