The present invention relates to a novel customized sRNA that reduces gene expression in prokaryotic cells, a preparation method thereof, and the use thereof, and more particularly to a synthetic sRNA comprising an Hfq binding site, derived from the sRNA of any one of MicC, SgrS and MicF, and a region that base-pairs with the target gene mRNA, and to a preparation method thereof and the use thereof. The synthetic sRNA according to the invention has an advantage in that the degree of inhibition of the target gene can be controlled by regulating the ability of the synthetic sRNA to bind to the mRNA of the target gene. The use of the synthetic sRNA that regulates the expression of the target gene makes it possible to effectively construct a recombinant microorganism without using a conventional gene deletion method and to reduce the expression of the target gene, and thus the synthetic sRNA is useful for the production of recombinant microorganisms. Also, the synthetic sRNA can be quickly applied to various strains, and thus is very suitable for the measurement of metabolic capabilities of strains and the selection of the most suitable strain. In addition, recombinant microorganisms, which are obtained by metabolic flux manipulation using the synthetic sRNA and produce tyrosine or cadaverine with high efficiency, are useful in the drug and industrial fields. In other words, the use of the sRNA according to the present invention can make it easy to select target genes whose expression is to be inhibited for the highly efficient production of metabolites. Accordingly, the synthetic sRNA can be used to construct recombinant strains for efficient production of various metabolites and to establish efficient methods for production of various metabolites, and thus is highly useful.