In this paper, we report the synthesis of dot- and branch-shaped Hg_xCd_1−xTe nanocrystals (NCs) with good stability and a high quantum yield of about 30% through an elaborate cation exchange reaction at room temperature. The large red-shifts in both absorption spectra and photoluminescence (PL) spectra confirmed the substitution of mercury ions for cadmium ions and the formation of Hg_xCd_1−xTe NCs. Interesting periodical XRD peaks observed in as-obtained Hg_xCd_1−xTe NCs indicated the formation of layered metal thiolates, which not only played a key role in introducing mercury ions during the cation exchange process, but also acted as ligands to maintain the emission stability of newly formed Hg_xCd_1−xTe NCs. The results indicate that the red-shift in PL emission has close-correlation with several parameters (such as the amounts of thiols and mercury ions, the sample store time, etc.).