The invention discloses a method for exploiting marine natural gas hydrates by using coupling frozen walls in a depressurization mode. The method comprises the following steps that (1), a refrigeration pipe net is led into a settled layer at the upper part of a marine natural gas hydrate reservoir with high natural gas hydrate abundance and under a seafloor; (2) refrigerating fluid is circularly injected into the refrigeration pipe net so that free water in the settled layer can be converted into ice, and frozen walls are formed around the refrigeration pipe net; (3) after the frozen walls areformed, exploitation in the depressurization mode is carried out on the hydrate reservoir under the frozen walls; and (4) exploited natural gas are stored and transported. The refrigeration pipe netis positioned at the upper part of the hydrate reservoir and under the seafloor and is close to a target hydrate reservoir, and the refrigerating fluid flows circularly in the refrigeration pipe net.After the frozen walls are formed, reservoir environment is not remarkably influenced. The method for exploiting marine natural gas hydrates by using the coupling frozen walls in the depressurizationmode can effectively prevent the sand plug phenomenon in a hydrate exploitation process by using a conventional depressurizing method and has great significance to achievement of natural gas hydrate deposit safety and efficient exploitation.