According to the actual distribution of solar energy in China, largescale photovoltaic power plants frequently connect to weak delivery AC systems, which have a low shortcircuit capacity but a high background harmonic content. The large capacity of photovoltaics connecting to the grid has already made a significant impact on the end grid’s damping characteristics, meanwhile providing a new method to suppress low frequency oscillation. Under the background, on the basis of the fourmachine twoarea system, an electromagnetic transient simulation model including largecapacity photovoltaic plantsis set up. And the low frequency oscillation characteristics of the system areanalyzed by the total least squaresestimation of signal parameters via the rotational invariance technique(TLSESPRIT). Aiming at coordinatelysuppressing low frequency oscillation in intraarea and interarea, the additional damping controller designed by linear quadratic regulator(LQR) control method with an observer isrespectively set in the different control loop of photovoltaic inverter and SVG. Simulation results show that the proposed method can effectively suppress the low frequency oscillation and increase the system's capability ofsending power in a certain extent.