Abstract: To quantitatively evaluate the risk of the liquefied natural gas(LNG) bunkering vessel with a cold box where LNG spilling, this study simulated the diffusion and deflagrations occurred at different instants after LNG spillage with or without a forced venting by fluid dynamics software Fluent.The simulation results showed that the flammable area increased first and then decreased with time after LNG spilling in the cold box without a forced venting when ignited at 60 s, 90 s, 120 s and 150 s respectively, the maximum overpressures could reach 29 kPa, 89 kPa, 76 kPa and 70 kPa correspondingly, which would cause serious damage to the box and persons.While with a forced venting, the methane concentration field internal cold box became stable soon after spill and the flammable area increased to a constant level.When the gas mixture was ignited at the same time as above, the overpressures were much less than those without a forced venting, as 1.8 kPa, 1.9 kPa, 3.0 kPa, 3.1 kPa respectively.And a fire was predicted resulting from the deflagration, which might cause heat radiation damage to the cold box and carried equipment.The method and results of the calculations could be applied in safety design of the cold boxes or other confined spaces to prevent the explosions and fires.