The selection, design and installation of common on-line moisture/viscosity/wear particle sensors are discussed. According to the characteristics of dry and wet lubrication systems of marine diesel engines, different design and installation methods of on-line oil monitoring sensors are proposed and tested. The results show that According to this method, the diesel engine online oil monitoring sensor designed and installed has good test stability, which not only meets the sampling requirements of oil online monitoring, but also greatly reduces the difficulty of sensor processing and installation.