The invention discloses an in-situ dynamic separation backfill device for submarine natural gas hydrate. The device is mainly used for solving the problems in the prior art that static backfill devices are limited in disposal amount, poor in separation effect, low in backfill speed and small in backfill amount. The device includes a power device, a dynamic sealing connector, a dynamic separation backfill system and a drill, wherein one end of the power device is connected to other hydrate exploitation dual-layer pipes and equipment, the dynamic sealing connector is connected to the other end of the power device, the dynamic separation backfill system connected to the dynamic sealing connector is in butt joint with the power device and can be driven by the dynamic sealing connector to rotate, and the drill is connected to the dynamic separation backfill system. According to the scheme above, the purpose of fast in-situ backfill is achieved, and the device has a high practical value anda high popularization value.