给出了 W型单模单偏振光纤基模截止的必要条件。通过对基模截止波长的数值计算 ,分析了这个必要条件的充分性。结论是 :对于圆形内包层 W型光纤来说 ,这是必要的而又充分的条件 ;对于低椭圆度内包层 W型单模单偏振光纤来说 ,它可以在较高的精度上近似作为充分条件 ,而对于高椭圆度内包层 W型单模单偏振光纤来说 ,它仅仅只是必要条件而非充分条件 The necessary conditions of the fundamental mode cutoff of W single mode single polarization fiber are given. By numerical calculation of the fundamental mode cut - off wavelength, the sufficient condition of this necessary condition is analyzed. The conclusion is that this is a necessary and sufficient condition for a circular inner cladding W-type optical fiber; it can be approximated with higher accuracy as a low-elliptic inner cladding W-type single-mode single-polarization optical fiber Sufficient conditions, and for the high ellipticity inner cladding W-type single-mode single-polarization optical fiber, it is only a necessary but not sufficient condition