Based on the adjoint simplectic subspace iteration method of gyroscopic system,a modal synthetic method of a large gyroscopic system(MSMGS) was proposed.This method divided the large gyroscopic system into several subsystems,and obtaned the first modals of the subsystems via the adjoint simplectic subspace iteration method.The whole transition matrix can be composed of the eigenvector matrices of all the subsystems,and the reduced gyroscopic system can be given through the whole transition matrix.The examples prove that the whole gyroscopic system can be approximated by the reduced gyroscopic system.