Butyrylcholinesterase (BChE) activity is an important index for a variety of diseases. In this work, a "turn-on" assay is proposed based on controlling the inner filter effect (IFE) of MnO(2)nanosheets (NSs) on sulfur nanodots (S-dots). The fluorescence of S-dots is effectively quenched by the MnO(2)NSs, due to the wide overlap of the emission spectrum of S-dots and absorption spectrum of MnO(2)NSs, together with the superior light absorption capability of MnO(2)NSs. BChE can catalyze acetylthiocholine and produce thiocholine, which effectively decomposes the MnO(2)NSs into Mn2+, resulting in the disappearance of the IFE and recovery of fluorescence of S-dots. Two-stage linear relationships between the ratio of fluorescence intensity and concentration of BChE are observed from 0.05 to 10 and from 10 to 500 U L-1. A limit of detection of 0.035 U L(-1)is achieved, which is the best performance so far. The as-proposed assay is robust enough for practical detection in human serum, and it can avoid interference from its sister enzyme (acetylcholinesterase) and glutathione at the micromolar level. The presented results provide a clue for the functionalization of S-dots, and offer a powerful tool as an analytic technique for nanomedicine and environmental science.