The present invention relates to a process of preparing liquid-phase hydrocarbon mixture, which comprises continuously conducting Fischer-Tropsch synthesis reaction in the upper part and hydrocracking reaction in the lower part by using a continuous-flow dual-bed reactor which comprises two fixed-bed reactors filled with catalysts and continuously placed in upper and lower parts and a hydrogen-supplying pipe between the upper and the lower fixed-bed reactors and continuously supplying hydrogen in the lower fixed-bed reactor, thereby remarkably increasing the yield of C
10 -C
20 liquid-phase hydrocarbon mixture as compared to the conventional Fischer-Tropsch synthesis reaction.