Accurate measurement of energy is a technical difficulty that various microseismic monitoring systems are facing. In this article, we applied the groundbreaking fiber optics microseismic monitoring system to mine rock burst monitoring for the first time around the world. We suggest to calculate microseismic energy via the duration of vibration. Precise picking of P wave first arrival and end point is essential to microseismic energy calculation. According to the huge energy difference before and after the arrival of the seismic wave, we use the sliding time-window energy ratio method to pick the first arrival and end point, and put forward the quantitative relationship between the sliding time-window width TWL and the sampling frequency as well as the signal dominant frequency for the first time. The results of P wave automatically picking are almost the same as those manually picked. However the end point may need to be corrected if wave distortion occurred.