Ballistocardiography (BCG) is a non-invasive method to detect the heartbeat signal, which reflects the body recoils introduced by cardiac ejection. Compared with traditional heartbeat monitors, such as Electrocardiography (ECG), the BCG detection method can acquire heartbeat signals without any wearable devices, which is user-friendly. The device can be integrated into a cushion, which is convenient for users to monitor the heartbeat in long-term. In this paper, a BCG monitor based on the Mach-Zehnder interferometer (MZI) with a new phase modulation method is demonstrated. A low-cost and compact phase shifter is introduced to solve the problem of signal fading in the fiber-optic interferometer. The baseline drift produced from breath and noise can be removed by the phase modulation and accuracy BCG signal can be detected in the sitting position. Compared with existing BCG monitors, such as accelerometer-based and bathroom scales-based, our optical fiber interferometer-based BCG monitor has many merits including sensitivity, stable, and immune to electromagnetic interference (EMI). In conclusion, the BCG monitor based on optical fiber sensor has great potential in the long-term and real-time heartbeat monitoring.