Abstract: In a PBS medium of pH 7.6 and in the presence of PVA, the fluorescence intensity (If) of phloxine at its fluorescence emission maximum of 555 nm was enhanced by addition of β-cyclodextrin (called “swtching in”), and was quenched by the presence of Ni(Ⅱ) ion (called “switching off”), and the magnitude of fluorescence quenching (ΔIf) was found to keep linear relationship with mass concentration of Ni(Ⅱ) in the range of 5-300 μg·L-1, with detection limit (3S/N) of 1.75 μg·L-1. This so-called fluorescence “switching” probe was used for fluorimetric determination of trace amount of Ni in waste water and ore samples, and recovery was tested by standard addition method, giving values of recovery and RSD′s (n=10) in the ranges of 98.8%-106% and 1.2%-2.2% respectively. A preliminary discussion on the mechanism of the fluorescence reaction system was also given.