Abstract: This paper presents a new, simple and fast algorithm of automated P wave detection in multi-lead ECG Signals. Range of QRS-T complex is detected firstly. Then QRS-T complex is eliminated. Finally, P wave's onset and offset are detected by using the average of low-pass difference method and tangent method. Tangent method is always used to locate the offset of T Wave but it will firstly be used to locate the P wave onset and offset in this paper. The proposed algorithm is tested by the annotated CSE database. Result shows that algorithm test result has a good consistency with BIS CSE annotation. Compared with the mean and standard deviation of P wave onset and offset, our algorithm and CSE annotation is -2.01 ms, 3.94 ms and 4.96 ms, 5.86 ms.