In this paper,by using the waveform data of medium and small earthquakes in Liaoning Province,we calculated the dynamic source parameters of 68 M_L ≥ 2.5 earthquakes from February 2012 to 2016,based on the Brune model. The results demonstrated that the range of apparent stresses of the Qingshiling earthquake swarm is 0-12bar. Apparent stress decreases gradually with the characteristics of rising and dropping rapidly. The stress drop increases with magnitude,which is consistent with the result gained by Nuttli that intraplate earthquake follows the ISD model. Compared with the scaling relationship between stress drop and magnitude of M_L ≥ 2.5 earthquakes in the 1999 Xiuyan M_S5.4 earthquake sequence,the stress drop of medium and small earthquakes in the Qingshiling earthquake swarm are smaller with the same magnitude apparently,about 10 times,that indicates fluid intrusion may play an important role in development and occurring of earthquakes.