Abstract: The optimal position( l _opt ) of isolating the heat conduction axially through the oscillatory tube wall is investigated in the present paper. The maximum moving distance( L _max ) of the contact surface inside the tube is studied and a calculating formula of L max is derived. Based on the formula of L _max , the influences of some factors such as expansion ratio, ratio of filling time and discharge time, number of nozzle, and jet flow exciting frequency on l opt are analyzed. It is shown that the value of l opt predicted by the formula is higher than that of experiments. The formula of L max can only be used to calculate the approximate value of l opt . The relation between lopt and the axial temperature distribution of the tube wall is also investigated. Both the theoretical and experimental results show that the position where the axial temperature gradient of the tube wall reaches the maximum value is exactly the position of l _opt So the value of l opt can be determined simply by means of measuring the axial temperature distribution of the tube wall.