Based on the isospin- and momentum-dependent transport model IBUU04, we investigated the neutron-proton differential flow in the 132Sn+124Sn mid-central collisions at beam energies of 400MeV/A, 600MeV/A and 800MeV/A by adopting two different symmetry energies. It was found that the neutron-proton differential flow as a function of rapidity is very sensitive to the density dependence of symmetry energy， especially at incident energies around 400MeV/A.