Abstract: By means of investigation of the pipeline operation characteristics under the condition of constant heat, the critical flow rate is found. When the flow rate is lower than the critical flow rate, the pipeline will be operating in a unsteady area.On the basis of analyzing the effects of the critical flow rate, it can be concluded that the critical flow rate under the condition of constant heat decreases with the decreasing of average overall heat transfer coefficient and heat increases with the increasing of oil temperature at the inlet in heating station.