Abstract: The free piston shock tunnel is one of highenthalpy ground facilities for hypersonic test, however the useful test time of this kind tunnel is very short. Increasing the constantpressure time of piston driver can be regarded as one of the available approach to extend the useful test time of the free piston shock tunnel. For this reason, the relation of the piston velocity and the constantpressure time in the piston driver was studied in this paper. The results revealed that the constantpressure time has local maximums with different velocity of piston and initial parameters, which form a ridge curve. Subsequently, the properties of this ridge curve were obtained by numerical analysis. It shows that the optimal or suboptimal design points of this tunnel should be close to the ridge curve, and its gradient of constantpressure time is small.