Due to the one-dimensional characteristic, the excitation of electrons in polymers will be self-trapped, and two excitons can be combined to form one biexciton. This combination process is one of the most important channels leading to form the biexciton, the efficiency of is much higher than the two-photon process. This kind of combination is accompanied by lattice distortion. By employing the dynamic equation, the relaxation process of this combination is investigated in this paper, and it is found that the relaxation time is about 160fs. The effect of external electrical field on the combination is also studied, the results show that when the electric field E is greater than 0.5MV/cm, instead of the biexciton, two bipolarons are produced.