In order to monitor temporal stress variation of underground medium，we built a signal transmitters based on the Controlled Accurate Seismic Source(CASS)near south part of Longmenshan Fault zone after the Wenchuan MS8.0 earthquake in 2008. 28 seismometers were set up to record the elastic waves emitted from the CASS. The source signal was detected in the records with low signal to noise ratio of the seismic stations，and transformed to seismic wavelet by matched filter technique. Then the SNR of the seismic wavelet was improved by stacking technique，and seismic wavelet of every day，every 5 days and every 10 days were obtained. In addition，the variation of seismic travel time was extracted by cross-correlation and the error range was assessed. Using CASS system is feasible for monitor variation of seismic travel time from the result. Finally，we compared and analyzed the variation of seismic travel time before and after the Mianzhu MS5.6 earthquake in 2009. The result showed that the earthquake with long distance had no effort on the seismic velocity of underground medium below our experimental region.