Objectives: Glyphosate was an efficient herbicide， while there was negative impact on water environment and human health. Therefore， it is significant to develop an efficient and accurate method to detect the glyphosate in water. In this study， a fluorescent turn-on probe was designed based on R-phycoerythrin in Porphyra haitanensis for detection of glyphosate in water. Methods: The crude extract in P. haitanensis was extracted by freeze-thaw cycle and ultrasonication. The peptide mass fingerprint， abosorbance spectrum and SDS-PAGE indicated the extract was R-phycoerythrin. The purity of R-phycoerythrin was as high as 4.96 after purification. Further studies indicated the R-phycoerythrin was specific fluorescence responsed to Cu2+ ions. A fluorescent probe based on the copper ion quenching R-phycoerythrin fluorescence was designed. Results: The probe fluorescence signal showed good linear relationship with the glyphosate concentration， and the detection limit was 443.4 μg/L， which was significantly lower than the maximum allowable residue limit (0.7 mg/L) specified for glyphosate in the Chinese National Standard for Drinking Water Quality (GB 5749—2022). The probe was used to detect the glyphosate content in two water samples and the glyphosate was tested. Conclusions: The study not only highly-value utilized of P. haitanensis， but also provides a new method for the detection of glyphosate residues in water， which has certain scientific significance and application value.

Porphyra haitanensis  /  R-phycoerythrin  /  copper ion  /  glyphosate  /  spike recovery test