Natural gas hydrates are considered a significant potential strategic energy resource for the 21st century. However, their commercial exploitation is still constrained by low production efficiency and reservoir instability. Among the available extraction methods, depressurization is regarded as the most practical approach. Nevertheless, the influence mechanisms of key engineering parameters, such as production well pressure and well length, on production performance and reservoir safety remain insufficiently clarified. Based on the actual geological conditions of the SH7 site in the Shenhu Area of the South China Sea, a numerical model for depressurization-induced production in a clayey-silt hydrate reservoir was established using the transport of unsaturated groundwater and heat + hydrate (TOUGH+HYDRATE) simulator. The effects of different production well pressures and well lengths on gas production performance, hydrate saturation distribution, and the evolution of reservoir cementation strength were systematically simulated and analyzed. The results indicate that in terms of production capacity, both decreasing production well pressure and increasing well length enhance the gas production rate and cumulative gas yield, although diminishing marginal returns are observed. Meanwhile, water production is significantly increased, and the gas-to-water ratio is reduced, which may negatively affect economic efficiency. In terms of reservoir mechanical behavior, a non-uniform inverted-cone-shaped expansion pattern of the hydrate dissociation zone is observed. Faster propagation occurs near the top and bottom boundaries, while slower progression appears in the central region. This phenomenon is mainly attributed to more efficient convective heat transfer near reservoir boundaries and limited conductive heat transfer in the central zone. The deterioration range of reservoir cementation strength, represented by parameter L, expands markedly with decreasing well pressure and increasing well length. Its spatial distribution does not completely coincide with that of hydrate saturation, indicating complex thermal-hydraulic-mechanical coupling effects. Overall, safe and efficient hydrate exploitation requires a balance between high productivity and reservoir stability. Excessively low production pressure or overly long well sections should be avoided to reduce engineering risks such as sand production, wellbore instability, and uneven formation subsidence. These findings provide a theoretical basis for optimizing engineering parameters in depressurization-based production of marine clayey-silt gas hydrate reservoirs.