This paper proposed a kind of new uncertainty reasoning mechanism called hybrid uncertainty inference model (HUIM). By this mechanism, the assumption based on the truth maintaince system (ATMS) and simplified Dempster-Shafer's evidence theory has been well combined. And as a result, the ATMS, which is originally viewed as only a symbol algorithm system, can be used to express uncertainty information in the manner of numerical values and can make it possible to calculate the numerical information under the combination rule of evidence theory. once this method is applied to rule -based on the systems, it would be hopeful that some weakness caused by the uncertainty reasoning of existed rule-based on the systems could be overcome.