The phase characteristics of an unbalanced Michelson interferometer based on a 3×3 coupler was investigated. Using the scattering matrix theory of fiber couplers, the phase differences between each two of the three outputs of the interferometer were deduced when an unideal 3×3 coupler was used. The scattering matrix of a commercial 3×3 fiber coupler was obtained from the insert loss of each channel after calculation and modification. The phase differences of the three outputs of the interferometer are calculated to be 120.21°, 120.77° and 119.02°, with deviation less than 1° from the ideal 120°. An experiment was carried out to measure the three outputs of the interferometer. Small fluctuation of the phase differences is observed, which is due to the random change of polarization state. The maximum of the deviations from the ideal 120° is within 1°, and the experimental results are in accordance with the theoretical expectations.