The first-principles calculation with the nonequilibrium Green's function formalism is employed to comprehensively demonstrate that the mechanical strain and anchoring group are two crucial impacts on spin transport in single-molecule magnetic junctions. For the dissociated amine-ended benzene contacted to cobalt electrodes, we present the strain-enhanced spin injection efficiency, including sign reversal and nearly perfect spin injection under junction stretching process. The underlying mechanism is the strain-assisted movement of pronounced and broad spin-up transmission feature toward the Fermi energy. This intriguing finding reveals the superior spin transfer in amine-ended single-molecule magnetic junction, which is in sharp contrast to the better charge transfer between gold electrodes in traditional thiol-ended molecular junctions. Our calculation results may pave the way for promising tunability of spin injection efficiency under mechanical stimulus of break junction technique.