As a key epigenetic modification,5-formylcytosine (5fC) controls many important cell functions,such as gene expression and cell differentiation.However,5fC exists in mammalian tissues and cells with low content,and its structure is highly similar to 5-methylcytosine (5mC),5-hydroxymethylcytosine (5hmC) and 5-carboxycytosine (5caC),which requires the detection method for detecting 5fC with high sensitivity and specificity.In this experiment,WS2/Bi/Bi2O2CO3 was used as the photoactive material,5fC an-tibody was used as the target molecule recognition reagent,and rolling amplification (RCA) was employed as the signal amplification strategy to construct a novel photoelectrochemical biosensor for detecting 5-formylcytosine deoxynucleotide (5fdCTP).Firstly,WS2/Bi/Bi2O2CO3 nanocomposite was prepared and characterized,and then modified on the ITO electrode.Then,polyacrylic acid(PAA) was immobilized by electrostatic adsorption,which was employed as the antibody capture reagent based on the covalent re-action of-COOH on PAA with-NH2 on Ab.Subsequently,5fdCTP was recognized through immunoreaction,triggering the subse-quent rolling circle amplification reaction with the aid of the crosslinker of Zr4+.Finally,positive methylene blue (MB) was adsorbed by negative ssDNA,resulting in an increased photoelectric conversion efficiency of WS2/Bi/Bi2O2CO3 and improved detection sensi-tivity.The linear range of the PEC biosensor for detecting 5fdCTP was 0.05-100 nmol/L,and the minimum detection concentration was 12.4 pmol/L (S/N =3).The applicability of this method has been evaluated by studying the influence of Cd2+ and antibiotic con-centration on the 5fC content in the genomic DNA of maize seedlings.