Determining an accurate first arrival time of P wave and S wave is very important in locating microseismic events,and manual picking is extremely time-consuming and subjective.An automatic two-step method based on after-term average over before-term average ratio(ATA/BTA) and the Akaike Information criterion(AIC) is developed.The ATA/BTA is used to detect the event and the AIC is utilized for picking.The method can attenuate the impact of window length and improve the precision of picking.The examination of the algorithm in different window lengths and various SNR environments by using the real data proves its effectiveness and practicability.