Abstract: When time reversal technique is used in phased array high intensity focused ultrasound system to obtain self-adaptive focusing,it is needed to calculate the initial phase of the sources.Because of the frequency shifting when the phase lock loop is initiated,different results are gotten in different segments using conventional methods.A new method of calculating the phase difference is presented.Because it can display the changes of phase difference in real-time,the steady phase differences can be regarded as the initial phase of the sources.The experimental results show that using the phase difference obtained by this method can get a better distribution of sound field in some spatial extent.