Isolator is a key component of Scramjet in achieving dual-mode and mode transition, and it separates the inlet from the combustion chamber, thus preventing the effect of combustion back pressure on the inlet and guaranteeing the security and stabilization of aircraft. Combined with the problem of combustion chamber work stability, a numerical simulation of axial symmetry isolators with same cross section is performed with FLUENT6.2. When the Mach number of the inflow is 2, isolator with different ratios of length to height is investigated and the influence of back pressure on the location and structure of the shock train and on the performance of the isolator is analyzed. The results show that the back pressure has ahead transmission on the shock train of the isolator, which results in the complexity of the shock train structure and the decrease of the outlet distortion degree. The ratio of length to height has a great influence on the correlation between the location of the shock train and the back pressure. If the ratio of length to height is not too high, the location of the first shock wave will have a linear relation with the back pressure, and the changing speed is in direct proportion to the ratio of length to height. Otherwise, the linear correlation will not hold true.