In this study, water-soluble PEG-modified Sr2YbF7:Tm3+ was prepared conveniently by a one-pot solvothermal method, where the molar ratio of Sr2+ to Yb3+ was controlled between 3 : 2 and 1 : 1. The optimum red-light emission at 677-699 nm was modulated via 0.7% Tm3+ doped under irradiation at 980 nm. Interestingly, biological experimental results showed that the PEG-modified Sr2YbF7:Tm3+ with low toxicity, excellent cell membrane permeability and high photostability can not only distinguish HeLa cells from mouse embryonic fibroblasts but also target the subcellular organelle lysosome in HeLa cells; therefore it can be anticipated that the as-prepared material would be a potential tool for cancer diagnosis.