The depressurization method is a common approach for the exploitation of natural gas hydrate reservoirs in marine areas. This method can induce complex multi-physical coupling responses in the near-wellbore reservoir， leading to pressure changes， temperature variations， hydrate decomposition， deterioration of reservoir mechanical properties， and formation subsidence. This study utilizes a fully-coupled hydro-thermo-mechanical numerical model to analyze the mechanical property deterioration and subsidence characteristics of marine natural gas hydrate reservoirs caused by depressurization in horizontal wellbores， characterizing the multi-field coupling response laws of the horizontal wellbore and surrounding reservoir， and identifying the influencing factors of reservoir mechanical property deterioration and subsidence. The simulation results indicate that the affected area of reservoir pressure and temperature changes is much larger than the decomposition front of hydrates， and the distribution of effective normal stress varies significantly in different directions. The deterioration area of cohesion induced by depressurization is highly correlated with the plastic zone and hydrate decomposition zone. The subsidence characteristics in the shallow and deep areas relative to the horizontal wellbore exhibites distinct features. The simulation results provide a reference for the stability analysis of marine natural gas hydrate reservoirs during depressurization exploitation through horizontal wellbores.