To suppress the low frequency oscillation and sub-synchronous oscillation in the interconnected system at the same time, a kind of modified projective control method is given. Oscillation modals and the reduced-order controller of the AC/DC transmission system operating in island mode are identified by total least squares-estimation on signal parameters via rotational invariance techniques. Taking advantage of the band-pass filters to decompose the different oscillation frequencies into different channels, then according to the modified projective control theory, multi-channel projective controllers are designed. The controllers can reduce the mutual influence among the different oscillation models, also, the order and robustness of controllers are taken into consideration. Considering the time delay problems when the wide area signals are in the course of gathering and transferring, which can cause the control results worse, the phase compensation controllers are designed based on the identification results of the system. Taking advantage of this method to carry on the modelling and simulation in one DC model under the island operating, then comparison and analyses are done with the system without supplementary damping controller. The results validate that the multi-channel projective controller can suppress the low frequency and sub-synchronous oscillation at the same time, also, good robustness is verified; the phase compensation controllers designed can solve the time delay problems effectively.