The M_S7. 3 earthquake occurred in Yutian,Xinjiang on February 12,2014. Based on seismic waveform data before the earthquake and aftershocks of the earthquake sequence,which were recorded by the Xinjiang Regional Digital Seismic Network, this paper corrected instrument response,propagation path and site response of the S-wave recording spectra. We then calculated with genetic algorithms,on the basis of the Brune model,the source parameters of the 102 M_L≥3. 0 Yutian earthquake sequence,seismic moment,apparent stress and corner frequency. The results show that,seismic moment of the earthquake sequence is between 3. 46 × 10~11- 2. 08 × 10~15N·m,apparent stress is between1. 48 × 10~5- 1. 16 × 10~6 Pa,mean stress level is 0. 31 MPa,and corner frequency is between1. 4- 7. 1Hz in the range of 3. 0- 5. 0. By analyzing the apparent stress and corner frequency variation with time,we obtain that apparent stress of earthquakes before the Yutian M_S7. 3 earthquake was significantly higher than the aftershock sequence,but the corner frequency was significantly lower than the aftershock sequence. Apparent stress was at a high level before the main shock, which shows that the main shock zone accumulated higher stress,and then the apparent stress was reduced. The main shock occurred in the process of slow increase. Because of the release of a large amount of stress,after the M_S7. 3 earthquake,the apparent stress was gradually reduced. That was the performance of low stress fracture of aftershocks. Based on seismic waveform data before the earthquake and aftershocks of the earthquake sequence, which were recorded by the Xinjiang Regional Digital Seismic Network, this paper corrected instrument response, propagation path and then site response of the S-wave recording spectra. We then calculated with genetic algorithms, on the basis of the Brune model, the source parameters of the 102 M_L ≧ 3. 0 Yutian earthquake sequence, seismic moment, apparent stress and corner frequency. The results show that the seismic moment of the earthquake sequence is between 3. 46 × 10 ~ 11 - 2. 08 × 10 ~ 15N · m, the apparent stress is between 1. 48 × 10 ~ 5 ~ 1. 16 × 10 ~ 6 Pa , mean stress level is 0. 31 MPa, and corner frequency is between 1. 4- 7. 1Hz in the range of 3. 0- 5. 0. By analyzing the apparent stress and corner frequency variation with time, we obtain that apparent stress of earthquakes before the Yutian M_S7. 3 earthquake was significantly higher t han the aftershock sequence, but the corner frequency was significantly lower than the aftershock sequence. the main shock zone was higher stress, and then the apparent stress was reduced. The main Because of the release of a large amount of stress, after the M_S7. 3 earthquake, the apparent stress was gradually reduced. That was the performance of low stress fracture of aftershocks.