We　present　a　finite　difference/spectral　method　for　the　two-dimensional　generalized　time　fractional　cable　equation　by　combining　the　second-order　backward　difference　method　in　time　and　the　Galerkin　spectral　method　in　space　with　Legendre　polynomials.　Through　a　detailed　analysis,　we　demonstrate　that　the　scheme　is　unconditionally　stable.　The　scheme　is　proved　to　have　min{2　-　alpha,　2　-　beta}-order　convergence　in　time　and　spectral　accuracy　in　space　for　smooth　solutions,　where　alpha,　beta　are　two　exponents　of　fractional　derivatives.　We　report　numerical　results　to　confirm　our　error　bounds　and　demonstrate　the　effectiveness　of　the　proposed　method.　This　method　can　be　applied　to　model　diffusion　and　viscoelastic　non-Newtonian　fluid　flow.