Abstract: This paper evaluates the power transmission efficiency of long magnetically insulated transmission line(MITL) and the effects caused by MITLs structure changes. The steady state flow theory and unsteady state flow theory of the MITL have been introduced. The particle-in-cell simulation and corresponding validation experiments have been conducted. The formula for Mendels arbitrary momentum model after laminar flow approximation are verified by the accurate measurement of the cathode current, anode current and interpolar voltage in the same position. Experiment results of the inductance-supported MITL show that by using the rigid spring of appropriate inductance to fix the internal and external tubes, the requirements of the transmission efficiency can be satisfied.