Water-soluble CdTe/ZnTe core-shell quantum dots (QDs) coated with L-cysteine were synthesized in low-temperature aqueous-phase one-pot approach. The authors measured the spectral characteristics of QDs at different pH in various buffer solutions and under different excitation laser powers. The primary results show that the absorption spectra of QDs approximately overlap and the fluorescence spectra peaks have no shift in different pH solution. The fluorescence intensity increased linearly with increasing pH. With the incubation time in borate buffer solution, the fluorescence intensity decreased a little. Under strong power laser, the QDs were photobleached rapidly. However, QDs are strongly anti-photobleaching under appropriate laser power (＜100 μW). Thus, such QDs have good biological stability and optical stability. By conjugating the QDs with transferrin protein and constructing the targeted fluorescent nanoparticles, the authors labelled the HeLa cell successfully. Photobleaching experiments in vivo show that microenvironment inside cells affect the stability and accelerate the photobleaching of QDs.