Abstract: This paper discusses a method of analysis and design in of a typical sample control system-direct digital control(DDC) system S and P domains.According to the theory of sample-data control systems, by starting from the stability criterion of digital control system in Z-plane and using tustin transform(p-transform) and the relations between Z-plane,S-plane and P-plane, the stabitity criterion for digital control systems in S and P domains is conducted.Further more,the analysis and design method digital control systemsin S and P domains is put forward.