We investigate theoretically the coherent conductance and magnetoresistance in a topological insulator ferromagnet/superconductor/ferromagnet (F/SC/F) junction with perpendicular magnetization. It is shown that there exists a large difference between the coherent conductance of the parallel and antiparallel magnetization configurations, which leads to a large magnetoresistance. The superconductor layer thickness dependence of the magnetoresistance is discussed in terms of the contributions of the local Andreev reflection, electron elastic cotunneling and crossed Andreev reflection to the parallel and antiparallel conductances.