Protein citrullination is an important posttranslational modification on an arginine residue. However, high quality fluorescent probes for measuring the citrullination level and capturing citrullinated proteins are quite limited. Inspired by the similarity between acid-promoted citrulline-labeling reaction and aldol reaction, here we present “turn-on” and “turn-off” fluorescent probes for measuring citrulline levels based on the scaffold of aldol sensors. Further application of the modified probe showed great potential to simultaneously monitor and capture citrullinated peptides.