This invention relates to a method of preparing an iron carbide/carbon nanocomposite catalyst containing potassium for high temperature Fischer-Tropsch (FT) synthesis reaction and the iron carbide/carbon nanocomposite catalyst prepared thereby, and a method of manufacturing a liquid hydrocarbon using the same and a liquid hydrocarbon manufactured thereby, wherein a porous carbon support is uniformly impregnated with an iron hydrate using melt infiltration, and potassium is also supported together via various addition processes, including a pre-addition process of a potassium salt which is ground upon impregnation with the iron hydrate, or a mid- or post-addition process of a potassium solution using incipient wetness impregnation after impregnation with the iron hydrate. Accordingly, the highly active iron carbide/potassium/carbon composite catalyst for high temperature FT reaction in which 5˜30 wt % of active iron carbide particles are supported on the porous carbon support can be obtained and is structurally stable to heat even in high temperature FT reaction of 300° C. or more, and liquid hydrocarbons can be selectively obtained at high yields.