An energy-efficient hydrate-based warm brine preparation method in situ seafloor was first proposed for marine natural gas hydrates (NGH) exploitation. The detailed preparation process and key technologies are discussed. The optimal hydrate-former, cyclopentane + CH4, is viable for preparing warm brine under various seawater depths. The heating coefficient of the warm brine preparation reaches 3.0. The NGH production performance by depressurization in conjunction with the prepared warm brine stimulation was studied by numerical simulation. The warm brine stimulation accelerates gas production. The gas production behavior performs better with the higher salinity and temperature. However, these positive effects are limited by the direct seepage of the brine from the injection well to the production well. The massive water production from the overburden and underburden layers causes low RGW and energy efficiency. Compared to the conventional hot brine injection, the good performance of the warm brine injection confirms the feasibility of the new method.