The authors measured the absorption and the fluorescence spectra of the quantum dots CdSe/ZnS with 4 nm in size at different concentration with the use of the UV-Vis absorption spectroscopy and fluorescence spectrometer. The effect of quantum dots CdSe/ZnS’s concentration on its fluorescence was especially studied and its physical mechanism was analyzed. It was observed that the optimal concentration of the quantum dots CdSe/ZnS for fluorescence is 2 μmol·L-1. When the quantum dot’s concentration is over 2 μmol·L-1, the fluorescence is decreased with the increase in the concentration. While the quantum dot’s concentration is less than 2 μmol·L-1, the fluorescence is decreased with the decrease in the concentration. There are two main reasons: 1) fluorescence quenching and 2) the competition between absorption and fluorescence. When the quantum dot’s concentration is over 2 μmol·L-1, the distance between quantum dots is so close that the fluorescence quenching is induced. The closer the distance between quantum dots is, the more serious the fluorescence quenching is induced. Also, in this case, the absorption is so large that some of the quantum dots can not be excited because the incident light can not pass through the whole sample. As a result, the fluorescence is decreased with the increase in the quantum dot’s concentration. As the quantum dot’s concentration is below 2 μmol·L-1, the distance between quantum dots is far enough that no more fluorescence quenching is induced. In this case, the fluorescence is determined by the particle number per unit volume. More particle number per unit volume produces more fluorescence. Therefore, the fluorescence is decreased with the decrease in the quantum dot’s concentration.