We have now shown that a host-guest complex between cucurbit[7]uril (Q[7]) and neutral red (NR) can be used as a fluorescent probe for detecting the herbicide paraquat. The Q[7]/NR complex can be used to detect paraquat with high selectivity using fluorescence spectrometry. A linear correlation (ΔF=4.98+0.32c, ｒ=0.999 1) was observed between the fluorescence intensity and the concentration of PQ over the concentration range (1~8)×10-6 mol·L-1. The detection limit for PQ was 1.40×10-8 mol·L-1. Recoveries obtained with the proposed method in river water examples were 104%~108%. Such method provides a rapid, selective and sensitive strategy for herbicide detection in real samples.