Abstract: The onset time picking is one of the most important procedures of ultrasound testing of rock, which is always a time-consuming effort. The picking precision is easily influenced by artificial factors. To improve the accuracy and the efficiency of acoustic velocity measurement of rock, the location of acoustic emission and ultrasonic tomography, the technique of AIC for automatic onset time picking, widely used in seismology, is introduced to the ultrasound testing of rock and is improved appropriately. In this paper, the onsets of simulated and actual signals are picked with AIC and improved AIC, respectively; and all the picking results are compared with the actual arrival time (just for simulated signals) and onset time picked by artificial and STA/LTA method. According to the experiment results, the AIC is superior to the STA/LTA method, for signals with low SNR; the AIC works better for signals with sharp onset and fast amplitude variation, and improved AIC is more suitable for signals with gentle onsets to pick the first arrivals; and the picking results of the latter are closer to the artificial results than the former.