Herein, two mitochondria-targeting photosensitizers (PSs, CCVJ-Mito-1 and CCVJ-Mito-2) that exhibit a turn-on fluorescence response towards increasing viscosity are reported. Notably, CCVJ-Mito-2 exhibits absorption in the near-infrared (NIR) region, and can be employed as a NIR PS targeting mitochondria and a fluorescent probe for tracking mitochondrial viscosity changes during photodynamic therapy (PDT). This dual functional PS can help to shed light on the dynamic changes of the cellular microenvironment during PDT and further guide the PDT process.