Abstract: To the confidence factors spreading in IFS approximate reasoning，the related formulas for finding the confidence factors of conclusions are exposed in this paper.First，with confidence factors been introduced，the effects of confidence factors spreading abroad in rules on those of conclusions are analyzed.To the IFL approximate reasoning with confidence factors，including the generalized modus ponens，generalized modus Tollens，generalized hypothetical syllogism on IFL，the related formulas are exposed.The correctness and validity of the proposed formulas are verified in a particular instance．