The waterproof roller sheet of modified asphaltum is extensively used in many waterproof constructions. The modified asphaltum by polymer is of good performance, such as cold-resistant, high intension etc. In the present paper, the modifying polymer was seperated from the waterproof membrane of modified asphaltum. The structure of the modifying polymer was identified by IR, 13C NMR, and Py-GC/MS. The results show that the polymers are SBS and polyisobutylene.