As the high voltage direct current（HVDC）system can be quickly controlled，DC power support can be used to damp low frequency oscillation by changing the power flow of the AC tie-line.The purpose of this study is to expatiate on the key technology of DC power support to damp low frequency oscillations，including DC line selection，support time and quantity of power support.It is assumed that the DC control sensitive factor is equal to the sum of products of the dominant generator and DC line power shift distribution factors.Based on the results of sensitive factor，the corresponding DC line is selected to support power.The accuracy and efficiency of this method is validated by the results of China Southern power grid.The opportune moment to support power is when the generator power angle starts to forward swing.The inhibitory effect on low frequency oscillation is not appreciable with a small quantity of power support，while a large quantity of power support will cause antiphase oscillation.There is no significant difference in the inhibitory effect on low frequency oscillation between the constant power and constant current control modes.The above results are obtained from theoretical analysis and time dominant simulation.