Polarization modulating effect was analyzed theoretically for pulling process of fused biconical taper fiber coupler. Geometrical and stressed birefringence caused different propagation constant for symmetric and antisymetric modes on x polarization and y polarization in coupling region. Therefore,it resulted in different coupling spatial period of two polarization orientation,with phase difference. The polarization beam splitting function was realized when the phase difference reached π. Also polarization beamsplitters were built in practice,with 16.5 dB extinction ration.