In photoisomerization polymer，the evolvement and some related properties of the dark stripe，in which the light field has a phase jump on both sides of the summetrical axis，are simulated by numerical method.The numerical results show that if the phase jump differs from π，the incident dark stripe will split into two grey stripes called Y-splitting, and the larger the phase jump (from 0 to π)，the smaller the Y-splitting.The numerical results also show that the angle between the phase-lead-branch and the direction of incidence is almost a constant for different phase jumps，while that of the phase-lag-branch varies with the phase jump.The angle between the two branches is unchanged under the condition of the same phase jump and different incident intensity.Besides，the numerical results also show that the refractive index change induced by Y-splitting in the material can act as a wave-guide，which leads an incident Gauss beam to form a Y-splitting.