Sensing and imaging pH inside living cells are of paramount importance for better penetrating cellular functions and disease diagnostics. Herein, we engineered an original pH sensor by a simple one-step self-assembly of poly(ethylene glycol) (PEG)ylated phospholipid (DSPE-PEG) and a phenol red small molecule on the surface of upconversion nanoparticles (UCNPs) to form a phospholipid monolayer for sensing and imaging the change of intracellular pH. The sensor showed excellent reversibility and rapid response to the pH variations. Furthermore, this pH sensing system could measure spatial and temporal pH changes during endocytosis and interrogate the pH fluctuations inside cells under external stimuli. Our experimental results revealed that the pH sensor was able to map spatial and temporal pH fluctuations inside living cells, showing its potential application in diagnostics and pH-related study of cell biology.