Selective and sensitive analysis of mercury (II) ions is a challenging topic in fluorescent sensors. Herein, we obtained a new selective mercury (II) ion fluorescence sensor by converting microscale hollow-tube-structured phosphorus-doped polymeric carbon nitride into phosphorus-doped carbon nitride oligomers with needle-like nanostructures by a hydrothermal reaction. The unique oligomeric microstructure and surface-rich terminal amino and hydroxyl functional groups of the fluorescence sensor synergistically endow it with selective quenching of mercury (II) ions. This phosphor-doped carbon nitride oligomer-based mercury (II) ion-selective fluorescence sensor achieves a minimum detection limit of 0.35 nmol L−1 in the range from 1–100 nmol L−1 (correlation coefficient of 0.9976) and a quantum yield of 16.6%. The detection of mercury (II) ions in actual shrimp samples demonstrated the sensitivity and accuracy of this fluorescence sensor. This work provides a new possibility for developing green and inexpensive mercury (II) ion-selective fluorescent sensors. Display omitted •Facile synthesis of nanoneedle-like phosphorus-doped carbon nitride oligomers.•A P–OCN fluorescence sensor based on selective quenching towards Hg2+.•The lowest detection limit of Hg2+ can reach 0.35 nmol L−1.