Temperature drop is commonly observed in subsea vertical X-mas trees during shutdown. The presence of a huge temperature difference between internal crude oil and external seawater can cause severe equipment degradation of the oil flow channel (e.g., hydrate precipitation), which can block the oil flow channel and interrupt the production process. The most vulnerable parts of a subsea vertical X-mas tree tend to be components with high convective heat transfer rates, such as production modules and short joints. We proposed an innovative approach for the insulation design of underwater equipment under a shutdown condition. First, we obtained a heat transfer analysis of the tree under working conditions through computational fluid dynamics to ascertain the initial temperature condition for an unsteady-state analysis. Second, we investigated the unsteady heat transfer characteristics of the tree with an insulation layer in the shutdown state and derived the relationships between insulation duration and thickness by data analysis. We used data analysis to identify the relationship between insulation duration and thickness. Finally, we derived the empirical formula of insulation thickness for underwater equipment given the effect of environmental factors on the heat preservation effect. We performed the experiment with an oil pipeline, and the results showed that the internal oil of the equipment did not hydrate within 8 h under the shutdown condition with insulation layers.