In the fields of automobile and other industries, there are many complex subsystems, which need to synthesize the inertia of different subsystems. The physical-measurement-based method has strict requirements for measuring equipment, long cycle and high cost. The CAD-measurement-based method has high requirements for model integrity and poor real-time performance. The theoretical-transformation-based methods are difficult to solve the composite transformation. Therefore, starting from the coordinate transformation of the position vector, the general form of the composite transformation of the translation and rotation of the inertial tensor component matrix is derived by using the matrix transformation, which further improves the theoretical transformation method. At the same time, the coordinate transformation problem of the inertial moment and inertial product is solved, the physical meaning of each item in the transformation relationship is given, and the theoretical results under different simplified conditions are discussed. The results of the typical example show that the theoretical method is consistent with the measurement results of CAD software, which verifies the effectiveness of the derived theoretical method. The relevant results can realize the rapid synthesis of inertia of multi component complex systems in engineering. Thus, the proposed method has certain theoretical significance and engineering application value.