The present invention discloses a tripolar VSC-HVDC system and method, wherein a rectifier and an inverter consist of a three-phase six-bridge arms modular multilevel converter (MMC) respectively, and two converter valves are arranged on the DC side of the rectifier and inverter respectively, the midpoint of upper and lower converter valves of the rectifier and inverter are connected with a pole 3 DC line by a smoothing reactor, triggering of the upper and lower converter valves is controlled to change the DC voltage polarity of the pole 3 periodically, and tripolar DC transmission is realized by modulating current orders of the three poles. The technical solution provided by the invention can make full use of the three conductors for power transmission, has the characteristics of small occupied space of the converter station, low total conversion cost and high reliability, and it is particularly suitable for the implementation of conversion an AC line to a DC line for capacity increasing in large-sized urban power supply systems lacking in land resources.