The dynamical evolutions of the order parameters and charge density distributions of self-trapping excitons in polymers under electric fields were obtained by a dynamical scheme. It was found that 1. Strong fields split the self-trapping exciton directly into a hole-polaron and an electron-polaron, which is consistent with the relevant experiments in polymers. 2. Weak or moderate fields polarize the self-trapping excitons, and the results show that absorbing one photon can induce a novel phenomenon of photo-induced polarization inversion.