On the assumption of stationary states, the effective mass theory is extended to the of superlattices in electric field. When the tunnel penetration factors are small, the calculation results are in agreement with those of tunnel resonance method. Subbands, wavefunc-tions and optical matrix elements are calculated. It is found that the shifts of particle densities and energies of the second excited states are opposite to that of ground states. In the electric field the overall shape of subbands is close to that without electric field but the double degeneracy is removed. With the increase of electric field, the selection rule △n=0 goes over to △n =±1 gradually, this change-over occurs at moderate electric field.