This paper provides a model-based method for processing the second-level uncertainties．In this paper，the authors use the space of probability class to present the second-level uncertainty structure of evidence space，L-set to present the second-1evel uncertainties of the rule strengths and use transmitting way of spaces of probability classes to describe the transmitting process of the second-level uncertainties．In this paper，the authors also present some transmitting properties of the specialization relations on classes of probabilities，and compares the two different ways of rule combination by using the concept of specialization．This paper gives a new comprehensive framework for knowl-edge-based reasoning．