The effect of an electric field on the subbands and excitons in a GaAs/Ga1-xAlxAs quantum well is studied by using a finite-potential-barrier model. The subbands of electrons and holes and the binding energies of excitons have been calculated for GaAs/Al0.34 Ga0.66 As quantum well (L = 105?) in electric field ranging from O to 1.2×105 V/cm. Based on these, the electron-hole overlap functions and the energy shifts of excitons corresponding to different subbands are obtained, they agree well with experimental results.