Traditionally the reduction of structure diagrams of complex control systems was realized by partitioning it into three basic types: serial, parallel and feedback connections. This method is always difficult and fallible in practice. Very often one have to move the positions of nodes and thus change the meaning of state variables. In this paper, a general matrix model for linear systems is deduced. According to this method, the computing model directly comes from the original structure diagram. With the aid of computing software, this problem can be solved very quickly and precisely.