Abstract: The marine gas hydrate in China has the hydrocarbon accumulation characteristics of small burial depth, thin layer, undercompaction and weak consolidation, and the effective mode for its industrial development in the future may be long horizontal well. In the process of drilling horizontal wells in marine gas hydrate reservoirs, however, cuttings carrying is not smooth for the hydrate reservoirs are undercompacted and weakly consolidated with very low rock mechanical strength and the hydrate bearing cuttings particles deposit under the action of gravity. And as a result, the friction drag and torque of the drilling tool are increased and the extension capacity of horizontal well is limited. The models for three hole enlargement modes were established, i.e., right-angle hole enlargement, 45° hole enlargement and arc hole enlargement. Then, coupling simulation was conducted using EDEM and FLUENT, and accordingly the theoretical chart of critical flow velocity needed for cuttings carrying was prepared. The simulation results show that in three hole enlargement modes, when the hydrate abundance is fixed, the critical flow velocity needed for cutting carrying increases with the augment of hydrate bearing cuttings particles, but decreases with the increase of drilling fluid density. When the drilling fluid density is fixed, the critical flow velocity needed for cutting carrying decreases with the increase of hydrate abundance. When the hydrate abundance and the drilling fluid density are fixed, straight-angle hole enlargement is the lowest in cuttings carrying capacity, 45° hole enlargement is moderate and arc hole enlargement is the highest. The research results provide the early technical support for optimizing the construction parameters in the exploitation mode of marine gas hydrate reservoir by horizontal well.