Abstract: Open Rack Vaporizer (ORV) and Submerged Combustion Vaporizer (SCV) are important LNG gasification equipments at LNG terminals. The operating cost of ORV is much lower than that of SCV, but the operation load of ORV is limited when the inlet temperature of seawater is low in winter. In this paper, the operation situations of ORV at the inlet temperature of seawater between 2 ℃ and 6 ℃ were tested, and the criterion for determining the maximum operating load of ORV was proposed. Then, the inlet and outlet temperatures of seawater and the LNG flow rate were fitted by means of software Origin, and the function for the maximum allowable LNG flow rate of ORV was established. By virtue of this function, the maximum allowable LNG flow rate of ORV at different inlet temperatures of seawater can be calculated accurately. Finally, the operation mode optimization plan of ORV and SCV was prepared by this function, combined with the sendout rates of one LNG terminal in 2015-2016 winter. Eventually, the gasification cost of the LNG terminal in winter was saved by about RMB10.7 million by making full use of ORV to the maximum extent for gasification.