The stability of soliton pairs and the electronic energy level structures in a doped poly-acetylene chain have been studied under natural boundary conditiono. In the Itamiltanian usel an end point potential was added to the SHH model, and the screened Coulomb potential of impurities and e-e interaetion on the tattice points were also indndelincluded. The caleulated results Show that, as the range of coulomb interaction is long enongh, stable soliton pairs are formel in the chain, and the stability is enlanced by the influce of the ends the gap width becomes narrow with the increase of chain length. But the deplndence of transition energg from soliton level to band bottom on chain length is not very signicant.