Abstract: A new design approach is proposed according to fuzzy control theory.It has been applied to designing the proportional and integral adaptive control device for a speed digital adaptive control system of a two-spool turbojet.Two methods (phase-plane method and membership degree method) have been used in this design.The digital simulation of the whole system has been completed.Its results demonstrate that this new approach brings the advantages of the proportional,integral and fuzzy controls into full play.It simplifies the control algorithm and enhances the control accuracy.Finally,this new design approach is suitable for nonlinear and high-order controlled object,which is lacking precision mathematical model.