Abstract: Using the data averaged 0ver one minute 0bserved by G0ES-7, the relation ofsoft X-ray burst character with source strength for six large Xray flares in Marehand June l99l has been studied. The result shows tha for the flares with intense softXray burst the fastor the flux increases with hme at beginning, the larger the flare-as-sociated shock velocity in corona is. The numerical simulation result reveals that therelease speed of energy may be the main index for a burst source.