The invention discloses an MMC-HVDC system, a DC side isolation device and an isolation method. The DC side isolation device comprises first isolation switches, current transferring switches, input/output terminals and a DC breaker. One terminal of the DC breaker is connected to the ground, and the other terminal is connected to all first isolation switches; each of the first isolation switches is connected to one input/output terminal through the current transferring switch; the current transferring switch comprises a first lightening arrester and a first IGBT group or an H bridge group, wherein either the IGBT group or the H bridge group is connected to the lightening arrester in parallel; the first IGBT group consists of N first IGBTs and N second IGBT groups; the H bridge comprises N H bridge circuits which are connected in parallel; and each of the H bridge circuit comprises a first capacitor and four third IGBTs, wherein N>=kU0/U1, E[element of]Z. The invention realizes the isolation between the MMC and the plurality of DC circuits and the MMC-HVDC system, and saves the investment cost of the power grid.