This paper deals with the problem of modeling for a mechanical system with a big number of degree of freedom. The problem becomes to be essential for a case when a mechanical system changes the number of degree during its processing and for the goal of control it is necessary to get a current mathematical model of an object. The paper contains the mathematical supply for automatic computer solution of this problem. On the base of this mathematical supply some results of the mathematical model decomposition are obtained. As an example for a mechanical system a space robotic mod-ule is considered