The MS7． 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，the 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 102 ML≥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 × 1011 ～ 2. 08 × 1015， apparent stress is between 1. 48 × 105 ～ 1． 16 × 106，and mean stress level is 0. 31MPa，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 MS7. 3 earthquake was significantly higher than the aftershock sequence，but the corner frequency was significantly lower than aftershock sequence． Apparent stress was high level before the mainshock，which shows the mainshock zone accumulated higher stress，and then the apparent stress reduced． The mainshock occurred in the process of slow increase． Because of the release of a large amount of stress，after the MS7. 3 earthquake，the apparent stress gradually reduced． That was the performance of low stress fracture of aftershocks．