To investigate the characteristics of an earthquake sequence of the Qinghai area,we fitted the aftershock sequence of the 9 earthquake sequences since 2009 in this region with the Epidemic-Type Aftershock Sequence(ETAS)model. The integrity magnitudes of nine earthquake sequences were estimated with the magnitude-rank and with the adopted different cutoff magnitude M_c. It is observed that we have to be cautious because the cutoff magnitude M_c has a certain effect on the value of α,k and p. There is no obvious relation between the stability time of the alpha value and the p value in the Qinghai area and the magnitude of the mainshock earthquake. The stability time of the sequence parameters may be shorter for the earthquake sequences with a larger mainshock magnitude. The earthquake sequences with small magnitudes of mainshocks may also have longer stability times. In the same earthquake sequence,there is no obvious correspondence between the stability time of alpha and P values and different cut-off magnitudes. Finally,the longest stable time and the shortest stability time of the parameters are given.