In the present paper, the authors studied the effects of anionic polyelectrolyte sodium polystyrene sulfonate (PSS) on the cationic dye thioflavin T (ThT) fluorescence spectra in aqueous solution. The influencing factors of the formed thioflavin T excimer in the existence of PSS were investigated. Experimental results show that PSS could increases the local concentration of ThT by electrostatic attraction, therefore enhances excimer formation of thioflavin T. The fluorescence intensity of ThT monomer and excimer (IM and IE) changes regularly along with the change in temperature; ln(IE/IM) and 1/T have good linear relationship. Hereby we have developed a new type of fluorescence ratiometric sensor for temperature which could be recognized by naked eye, and it could effectively eliminate environmental effects such as instability of exciting light intensity and photobleaching， therefore improve the accuracy of identification.