On the requirement of flexible logic of intelligent decision support system to classic logic, The flexible logic based on universal logic is proposed. Firstly the requirement of flexible logic of intelligent decision support system is analyzed. Then the method and theory of universal logic is given. The knowledge induction method based on universal logic is proposed. Lastly the framework and principle of intelligent decision support system based on universal logic is given. The problem of applicant on universal logic in intelligent decision support system is analyzed.