As a kind of folk medicine, edible mushrooms are known to be of medicinal characteristics, for example antitumor activity. However, the mechanism is not clear. In this study, the fluorescence (FL) spectroscopy was used to study the binding reactions of polysaccharide extracted from Auricularia polytricha with quantum dots (QDs). The modified Stern-Volmer quenching constant at different temperatures and corresponding thermodynamic parameters were calculated (a dagger G < 0, a dagger H < 0, and a dagger S < 0). The results show that the quenching constant is inversely correlated with temperature. It indicates that the quenching mechanism is the static quenching in nature rather than dynamic quenching. The negative values of free energy (a dagger G < 0) suggest that the binding process is spontaneous; a dagger H < 0 and a dagger S < 0 suggest that the binding of polysaccharide to QDs is enthalpy-driven. QDs were found to damage the plasmid DNA according to atomic force microscopy images. DNA damage is the important factor to induce tumor. Many chemical substances can induce tumor, which has been proved by modern medical science. In the presence of polysaccharide from A. polytricha, however, DNA was protected from damage due to polysaccharide winding around QDs, which is the basis for the bioeffect of polysaccharides. This study helps to understand that antitumor activity of edible mushrooms is attributed to protection of DNA from damage in the presence of harmful substances.