Abstract: An in situ measurement method of sound speed in liquid for acoustic emission testing of cylindrical storage tank was proposed, and some factors influencing the precision were also analyzed by an experiment. The approach was based on the normal acoustic emission testing scheme, and a simulation source was used to stimulate vibrations which could be detected by some sensors, finally the sound speed was conducted by computing the transportation time of the first wave directly arriving at through liquid. In the following experiment, three simulation signals, namely break pencil, 30 kHz and 60 kHz,were compared to find that the pulse of high frequency is the best choice on locating the first wave directly arriving at through liquid.The experiment revealed that the envelop-tangent method was superior to the other two methods (the threshold method and the cross-correlation method) in computing the time difference of arrival waves. The study also showed that the accuracy of this in situ measurement method was very high.