Debris analysis is one of the important methods for the health monitoring of marine power equipment,but marine lubricants are seriously degraded due to pollution,which affects the validity of ferrograph debris analysis.In order to verify the effective boundary conditions of online ferrograph in the monitoring of wear debris of marine engines,taking the No.3 oil pan lube oil in the No.3 dredger of the Yangtze River Waterway Bureau Suction as a sample,the failure boundary of the online ferrograph system was determined through analyzing the deterioration degree of the lubricant.Oil samples of different degrees of pollution were collected,and two sets of images of undeposited abrasive grains and deposited abrasive grains were obtained by online ferrograph.Through the analysis of the wear debris images,the influence of the wear debris concentration on the segmentation accuracy of the wear debris images was studied,and the relationship between wear debris concentration and the effectiveness of ferrograph wear debris image was obtained,and the testability evaluation method of ferrograph image of brightness and average gradient was constructed.The experimental results show that both methods can effectively separate the effective and invalid pictures,which lays a foundation for the reliability analysis of the iron spectrum picture and the realtime online analysis of the iron spectrum.