Abstract: The reason why unusual flow signals exist and the main effect of the siguals were investigated in hydraulic systems. It is pointed out that flow signals are also a rich carrier of condition information after the examination of the close relationship between the work condition of hydraulic systems and the flow nals. According to the experimental research on the faults of gear pump, it is practicable and effective using flow signals to monitor the condition of hydraulic systems and to diagnose the faults.