The ionochromic properties of Crystal Violet Lactone(CVL), 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalate with metal cations were investigated. It showed interestingly ring open reaction with addition of several metal ions in $CH_3CN$ solution. Upon addition of $Hg^{2+}$, the CVL shows a new peak around 603nm in its absorption spectra, and the color of solution changed from colorless to blue. Whereas other ions including $Fe^{3+}$, $Cu^{2+}$, $Fe^{2+}$, $Al^{3+}$ and so on induced basically weakly spectral change, which makes it promising application in $Hg^{2+}$ sensor.