The behaviors of the electroluminescent polymers under applied electric fields were studied by using the tight binding model of electron lattice coupling and solving the real tmie Newton dynamical equation. It was discovered that (1) the extra electron and hole injected by the applied electric field make the lattice relaxation, and bipolaron excitons are formed. (2) there exists a critical electric field. When an applied field is equal to or larger than the critical field, the bipolaron exciton will be dissociated into positive and negative polarons, leading to photoluminescence quenching. The result is consistent with the relevant experiments, and it also implies that the bipolaron exciton is a kind of luminous species in polymers.