The invention discloses a device and a method for exploiting natural gas hydrates of seabed superficial-layer non-diagenetic stratums on the basis of a vacuum voltage drop method. A vacuum pump, a water-gas separation tank, a grouting system, a heat injection system and a gas storage device of the exploiting device are connected through pipelines, and are fixed to an exploiting boat. The exploiting device contacts with a seabed natural gas hydrate stratum. During working, the exploiting device is compressed into a natural gas hydrate reservoir stratum, and negative pressure is generated in aninner sleeve pipe through the vacuum pump. When the hydrates are decomposed, water in an exploiting region of the hydrate reservoir stratum is gradually drained under the action of a water separationlayer formed through external sleeve pipe isolation, press plate movement and jet-hole grouting, a water supplement channel outside the exploiting region is blocked, the exploiting region is solidified gradually, and the hydrate reservoir stratum is reinforced while the hydrates are exploited. The device and the method have the advantages that geological disasters such as reservoir stratum unstability and collapse and seawater poisoning are solved, and stability of seabed structures is guaranteed.