Abstract: The Fresenius 4008 S hemodialysis machine is one of the main hemodialysis machines in our hospital. Because of its longer service life, the probability of malfunction is larger. For flow alarm is the typical fault of this model, we combined with practical maintenance experience and theoretical technology, then briefly analyzed and summarized the malfunction. On the principle of easy to difficult, we check in order from five aspects, they are pipeline blockage, aging of the degassing pump or flow pump, the float switch malfunction, valve abnormality and the control circuit problem. Through this one-by-one and systematic troubleshooting, the flow alarm can usually be solved, also we can save the maintenance time and the cost.