A new algorithm for calculating oscillatory active power increment distribution in power systems is proposed. The algorithm demonstrates the dominant effect of generators on the whole system oscillation increment distribution. A simple relationship is derived between a generator's power oscillation increment with its inertia and right eigenvector. The algorithm is verified by analyzing a typical low frequency oscillation in China. Furthermore， the mechanism of large scale of power oscillation on inter-connection lines initiated from local plant mode is studied from the aspect of oscillatory active power distribution.