Equations of motion for such multibody flexible structures as space structures containing a large number of degree of freedom is formulated by using a system written in a Object-Oriented language C++. This paper aims to achieve the following two accomplishment: The first is a general formulation of the equations of motion for multibody flexible structures applying the extended Hamilton's principle; The second is the development of the system for the formulation and the multibody structures are analyzed by utilizing the approach of Object-Oriented analysis. The developed system constitutes a mathematical simulation program for dynamical system and is formatted in C/C++ or Mathematica or Matlab.