Abstract: The discreteness of 70% energy duration caused by different threshold values is large. It will lead to be inconsistent and difficult to apply in engineering. Records of Wenchuan earthquake were chosen to analysis the difference of 70% energy duration defined by the threshold values between from 5% to 75% and from 15% to 85%. The spatial variability law of 70% energy duration with fault distance was also discussed. The results indicate that the 70% energy duration defined by the threshold value from 15% to 85% often includes much larger time region which is not strong vibration, while the 70% energy duration defined by the threshold value from 5% to 75% represents the time region of the strongest and the most destructive vibration. 70% energy duration based on the two definitions of threshold values will increase with the fault distance increasing. By contrast, the 70% energy duration from 5% to 75% fitting fault distance is better than that from 15% to 85%. In view of Wenchuan earthquake records, suggested using total seismic energy between 5% and 75% as threshold to definition of 70% energy duration.