In this paper, the mechanical model of the stepped piston vibrator is constructed and the generating mechanism of impact signal produced by the vibrator is studied firstly. Then based on experiments, the exciting signals produced by two common kinds of stepped piston vibrators are emulated under different pressure values. Finally the low frequency energies of exciting signals are taken as the performance indexes of vibrators and two kinds of vibrators are evaluated. The results of this paper show that big vibrators are superior to small ones under different pressure values.