Polarization fluctuation in polarization maintaining fiber (PMF) resonator caused by cross-polarization coupling is one of the major noise sources in resonant fiber optic gyroscope. Polarization axis 90° butt joint is an effective way to overcome the polarization fluctuation, and the size of butt docking error has important effect on the gyroscope noise suppression effects. The 90° butt joint error control of double-coupler PMF resonator based on Mickelson white-light interfero meter is studied theoretically and obtains the interference formula of polarization mode, and then the butt joint error degree. Finally, 90° docking error control in two-coupler PMF resonator is studied in experiment, and the experimental butt joint error of 0.37° is obtained.