The frequencies of natural oscillation mode in large power system are very close. When the system parameters experience minor changes， system has the potential to instability of close modes. A perturbation technique for complex modes is used to make modal analysis of the close modes shapes in a power system. It shows that eigenvalues are not sensitive to the perturbation while the left eigenvectors and right eigenvectors of close modes change a lot after the perturbation of system parameters. This will directly affect the controllability and observability of state variables on the oscillation modes. The 10-machine New England and NPCC 48-machine test systems are employed to verify the conclusion.