By taking the dielectric constant and viscosity as objects of performance monitoring of the hydraulic oils in engineering machinery, the online monitoring system of fluid performance of hydraulic oils based on FPS fluid properties sensor was established. The influence of temperature and pressure on the testing results of sensor such as the dielectric constant and viscosity was investigated,and the influence of water content and aging of oils on the dielectric constant of oils was analyzed. The static online monitoring system of engineering machinery hydraulic oils was designed to provide monitoring of the oil in the hydraulic system of the actual vehicle. The monitored results are accorded with that of the laboratory sampling testing.