Abstract: Path imbalance measurement of the two arms fiber optic interferometer includes employing a current carrier signal to modulate the semiconductor laser light source of the interferometer to let it output interference signals to generate a carrier phase signal through path imbalance of the interferometer. Then, the interference signals are expanded to be the harmonic components of carrier phase signal frequency by Bessel function. Subsequently, we use the specific relation between the second and the fourth harmonic components of the interference signals to develop the theory of path imbalance measurement. The method mentioned above can measure a few decimeters of path imbalance and its accuracy can reach to a millimeter.