Abstract: Aiming at the imprecision and uncertainty of risk factors，this paper analyzes the basic characters of IT project，and points out the problems existing in the group decision making．Meanwhile，the basic steps of RSGDS is put forward． With the rough set theory，the main processes of knowledge reasoning such as attributes reduction and rules mining are discussed，especially introducing the concept of dynamic weight，then the algorithm of rules reduction and mining is optimized and ameliorated．