All optical switching and excited state proton transfer(ESPT) effect of 7-hydroxyquinolin(7-HQ) in dimethyl sulfoxide(DMS) solution were investigated.7-HQ presents ESPT reaction by the excitation of ultraviolet light when it is solved in some solvent,which can form intermolecular hydrogen band.And that leads to the change of the molecule structure and the refractive index of solution.Therefore 7-HQ is an excellent material for all optical switching.However,7-HQ in DMS solution cannot present ESPT reaction because the intermolecular hydrogen band can not be formed between 7-HQ and DMS.So the solution of 7-HQ in DMS has not all optical switching effect.But in our experiment,we observed that this solution presented all optical switching effect after it was radiated a few minutes with strong ultraviolet light.By comparing the absorption spectrum and fluorescence spectrum of the 7-HQ in DMS solution,which was radiated with ultraviolet light and without,the mechanism is investigated.