The operating principle and character for optical splitter with polarization are analyzed by using tunable Faraday's rotator, the expression in ordinary and extraordinary light of coupling ratio of optical splitter are obtained, and the calculated curves for φF～D, φF～GR.(CRe),CE0(CRe)～D relation parameters are givea The precision adjusted for quantity of displacement can attain to μm order of magnitude when intensity of magnetic field H=79580 A/m. The coupling ratio to optical splitter will tune continuously in 0%~100% range, and it will be provided with two functions on splitting as well as attenuation.