Random mode hopping in narrow line-width lasers is a significant factor that affects the stability of the optical system. Narrow line-width lasers always have long resonantor length and small intervals between two neighbor modes. A new method of detecting mode hopping based on unbalanced fiber interferometer for very narrow line-width laser is proposed. It transforms the mode hopping process to the phase change of the unbalanced interferometer. Phase generated carrier (PGC) modulation-demodulation technique has been used to detect the phase signal. This method has high sensitivity and can be used in long time and real-time observation with better effects than Fabry-Perot interferometer. It can also be used in measuring the line-width (～kHz) of the laser, and gives a good testing method to improve the performance of the laser source.