In order to satisfy the demand of scramjet ground test, a gas generator was designed. Tests were taken to study the stability characteristics of combustion using the statistical approach. Results indicate that the proportion of unsteady combustion accounts for 32.7%. All unsteady combustion occurred belongs to low frequency oscillation. There are two kinds of oscillation forms. One has dominant frequency crest, which has low frequency combustion instability or chuffing. For the other one, there is no dominant frequency crest，namely rough combustion. The probability that rough combustion occurs is relatively high and it is related to the excess oxidizer coefficient, while the low frequency combustion instability occurs with lower probability. Result shows that the energy distribution of the two unsteady combustions is different. 