Abstract: In free piston shock tunnels, the non-ideal opening behavior of the main diaphragm may have different effects on the free piston motion and the shock wave formation. Based on the diaphragm shear-strain model and piston dynamic model, the coupled equations involving piston motion and diaphragm rupture are obtained for the first time, and the effects of the diaphragm opening process on piston motion and driving time under constant pressure are described. Research shows that, due to the interaction between the piston front pressure and the piston speed, the speed of diaphragm rupture or rupture ahead of time has little effect on the free piston terminal speed, and the piston can achieve soft landing at the end of the compression tube. This conclusion is confirmed by test results. Moreover, the test results also show that the non-ideal opening behavior of the main diaphragm has a distinct influence on the formation of the shock wave, and it gradually weakens during the propagation of the shock wave.