The technology of ECG (Electrocardiograph) auto-analysis has developed rapidly in recent years. There are lots of related techniques appearing. The majority is about R-peak detection. Although P-peak is important too, there are a few kinds of method about P-peak detection. In this paper is presented a new method for P-peak detection. According to Prony analysis, ECG is separated first, and then detected. The method gets good result. Data in experiment are from MIT Arrhythmia Database. The reliability of this algorithm in accurately detecting the P location is 97.8%.