The precursory characteristics of moderate earthquakes in Gansu province and its surrounding areas were studied by the region-time-length algorithm improved by Jiang Haikun and others. The result showed that this method could be used to detect the anomalous seismic activity of this area, for it had a good effect in retrospective test on monitoring earthquake. In the 36 M_s≥5.0 earthquakes, 24 earthquakes of abnormal RTL value had been detected, accounting for 66.7% of total. The intervals between abnormal time and time of main shock were centered on 10~37 months.