In view of the stability problem that the optical fiber Mach-Zehnder interferometer (MZI) is easily impacted by the environment, an MZI experimental system was designed. The output intensity of the optical fiber MZI was analyzed theoretically and validated by experiment. The relationship between the stability of MZI and environment and the impact of phase modulation on the interference were also revealed intuitively by oscilloscope. A new phase compensation scheme was designed to overcome the instability which results from vibration noise, temperature variation and so forth. The scheme can realize phase difference detection and feedback control.