The effects of free‐carrier screening and valence band mixing on gain‐switching characteristics of the field‐effect quantum‐well laser are studied theoretically. Our analysis is based on the multiband effective mass theory and the density matrix formalism with the intraband relaxation taken into account. We calculate the electronic structure of the quantum well by solving simultaneously the multiband effective mass equations for the envelope functions with Poisson’s equation. It is found that the free‐carrier screening affects considerably both electronic properties (band structure and wave functions) and gain‐switching characteristics by reducing the electric field and charge separation in the quantum well when the gate field is applied across the quantum well.