The invention relates to a flexible direct current converter station submodule auxiliary replacing apparatus based on a MMC and a replacing method thereof. The auxiliary replacing apparatus comprises a slide seat, limiting components and a rolling component. The limiting components are arranged at two side edges of the slide seat. The limiting components and the slide seat cooperatively form a slideway, wherein a submodule slides on the slideway. The rolling component is movably arranged on the slide seat and a part of the rolling component is exposed in the slideway. The rolling component can roll relative to the slide seat. Through using the flexible direct current converter station submodule auxiliary replacing apparatus based on the MMC, during a replacing process of a converter valve submodule, a new submodule can be rapidly and accurately installed at a valve tower submodule installation position from a work platform so that a requirement of submodule replacing work to personnel body quality is reduced; converter valve submodule replacing efficiency is icnreased; and power failure time is reduced and economic benefits are increased.