Due to the advantages on small size, light-weight, compact structure, immunity electronmagetic interference and high sensitivity, fiber Mach-Zehnder interferometer has been considered in various applications. However, the abilization of the fiber Mach Zehnder interferometer has recently become the most important problem to the measurement and communication system with higher accuracy and performance, because its non balanced parallel structure. A stability control system of the fiber Mach- Zehnder interferometer was introduced,which was based on 3dB optical coupler. It was realize the stability control by bias control, the output signal of microwave photonic link feedback to one of the fiber Mach Zehnder interferometer arms. This simple method was effective to overcome the unstably problem of the fiber Mach-Zehnder interferometer which composed of 3dB fiber coupler.