The flow field and coherent structure of the flow over a backstep with slot-gas-injection were investigated. The mean flow, turbulence properties and coherent structure characteristics are significantly influenced by the increase of injection momentum near the step (/r/H=2). The mean flow field is characterized by recirculation zone splitting, with mass transfer of injected gas toward each part depending on the specific momentum ratio of injection. More mass entrainment, turbulence structures alteration in the shear layer and the hydrodynamic disturbances from the injection flow suppress the coherent structure velocity, size and detachment. These results provide basis information for achieving the balance of competing factors between geometrical and fluid dynamical parameters of injection to establish an efficient design of a combustor from the perspective of enhancing the gas-air mixing process.