The oil contamination detecting instruments based the method of filter blocking have lots of faults,such as narrow test range,low integration,low intelligence and so on.In view of above problems,the online oil contamination detecting system was designed based on filter blocking with the application of computer and mechanical and electrical integration.The system consists of following devices,the sampling device with negative pressure,test device with constant pressure,filter backwashing device,PLC and ARM processor.The system can realize the following functions:online automatic sampling，automatic detection,and the test results are reliable.