A cascaded converter station for use in cascaded multi-terminal HVDC power transmission, comprising: a low-voltage end converter station having a positive side and a negative side, each of which comprising: a converter transformer (111a,111b) coupled to a first alternating current network; a converter valve (112a,112b) coupled to the converter transformer (111a,111b) for realizing DC/AC conversion; and smoothing reactors (115a,115b) provided on both ends of the converter valve (112a,112b); and a high-voltage end converter station, which is connected in series to the low-voltage end converter station through a middle-voltage DC power transmission line, and is connected to a high-voltage DC power transmission line, wherein the high-voltage end converter station comprises a positive side and a negative side, each of which comprising: a converter transformer (121a,121b) coupled to a second alternating current network; a converter valve (122a,122b) coupled to the converter transformer (121a,121b) for realizing DC/AC conversion; and smoothing reactors (125a,125b) provided on both ends of the converter valve(122a,122b); wherein, in the operation of the station, the DC voltage generated by the high-voltage end converter station is superimposed with the DC voltage outputted from the low-voltage end converter station to generate HVDC, which is then transmitted to the receiving side of the HVDC power transmission system, wherein the low-voltage end converter station comprises a grounding line coupled to a grounding electrode and a metal return line coupled between a positive line and a negative line, and the high-voltage end converter station comprises a grounding line coupled to the grounding electrode and a neutral bus isolation knife switch (137a, 137b), wherein one end of the neutral bus isolation knife switch (137a, 137b) is contacted to the grounding line, and the other end is contacted to the positive or negative output side of the high-voltage end converter station. a low-voltage end converter station having a positive side and a negative side, each of which comprising: a converter transformer (111a,111b) coupled to a first alternating current network; a converter valve (112a,112b) coupled to the converter transformer (111a,111b) for realizing DC/AC conversion; and smoothing reactors (115a,115b) provided on both ends of the converter valve (112a,112b); and a converter transformer (111a,111b) coupled to a first alternating current network; a converter valve (112a,112b) coupled to the converter transformer (111a,111b) for realizing DC/AC conversion; and smoothing reactors (115a,115b) provided on both ends of the converter valve (112a,112b); and a high-voltage end converter station, which is connected in series to the low-voltage end converter station through a middle-voltage DC power transmission line, and is connected to a high-voltage DC power transmission line, wherein the high-voltage end converter station comprises a positive side and a negative side, each of which comprising: a converter transformer (121a,121b) coupled to a second alternating current network; a converter valve (122a,122b) coupled to the converter transformer (121a,121b) for realizing DC/AC conversion; and smoothing reactors (125a,125b) provided on both ends of the converter valve(122a,122b); a converter transformer (121a,121b) coupled to a second alternating current network; a converter valve (122a,122b) coupled to the converter transformer (121a,121b) for realizing DC/AC conversion; and smoothing reactors (125a,125b) provided on both ends of the converter valve(122a,122b); wherein, in the operation of the station, the DC voltage generated by the high-voltage end converter station is superimposed with the DC voltage outputted from the low-voltage end converter station to generate HVDC, which is then transmitted to the receiving side of the HVDC power transmission system, wherein the low-voltage end converter station comprises a grounding line coupled to a grounding electrode and a metal return line coupled between a positive line and a negative line, and the high-voltage end converter station comprises a grounding line coupled to the grounding electrode and a neutral bus isolation knife switch (137a, 137b), wherein one end of the neutral bus isolation knife switch (137a, 137b) is contacted to the grounding line, and the other end is contacted to the positive or negative output side of the high-voltage end converter station.