The ultrasonic wear debris detecting technology has the advantages of high sensitivity, strong penetration ability, low cost, etc. It is a potential technology in the field of the online oil monitoring of the equipment. A set of ultrasonic online wear debris monitoring (UOWDM) system was designed based on VC++ program. The hardware system consisted of the focusing ultrasonic transducer, ultrasonic-testing card, computer and other supplementary devices. The software system consisted of the signal acquisition module, data processing module and monitoring result displaying module. The functions of real time acquisition of wear debris ultrasonic scattering signal, data processing and results displaying were achieved based on the developed UOWDM system. The experimental results show that the UOWDM system can monitor the oil wear debris online preferably.