In the present paper, the experimental and numerical investigations are described to clarify how the axial compression of the extension nozzle influences the unsteady flow fields and side load in startup and shutdown transients for a 2D dual bell nozzle. In the experiments, the flow fields were visualized and recorded by the schlieren method and the high-speed camera, respectively. Side loads were measured by a strain gauge. The flow visualizations show three flow patterns, i.e. an attached flow, an axial flow and a full flow, and the transients between the two flow patterns of those three. The strain gauge measurements indicate that side loads are produced by asymmetric transitions between the two flow patterns of the three The numerical results show the qualitative agreements with the experimental ones. They indicated that the jet oscillations are produced by vortices discharged from the extension nozzle exit alternatively and cause side loads. The axial compression of the extension nozzle changes the speed of the separation point movement.