A smart two-dimensional photonic crystal (2DPC) hydrogel was developed for accurate detection of Hg²⁺ by combining the Debye diffraction effect of 2DPC and the volume phase transition characteristic of hydrogel. The polystyrene (PS) 2DPC arrays were first fabricated by needle-tip-flow method, in which the monodispersed PS microspheres were orderly self-assembled on the water surface to form PS 2DPC and then transferred to glass slide. The poly(acrylamide-allylthiourea) hydrogels embedded with the PS 2DPC arrays were fabricated by “sandwich” method, in which the polymerization precursor solution including monomers acrylamide and allylthiourea was added to the PS 2DPC surface and covered with a glass slide, followed by photopolymerization. The atoms including N, O and S from functional groups on the hydrogel chains selectively coordinated with Hg²⁺, shortening the distance between the hydrogel chains and increasing the crosslinking density of the hydrogel. As a result, the hydrogel shrank and the PS microsphere spacing of the 2DPC decreased, which increased the diameter of the Debye diffraction ring. The microsphere spacing changes were acquired by monitoring Debye diffraction ring diameters before and after the response of the 2DPC hydrogel, achieving the highly sensitive, selective, reversible and fast detection of Hg²⁺. The linear detection ranges were in 10~100 nmol/L and 25~200 μmol/L, and the limit of detection was found to be 3.32 nmol/L. The prepared smart 2DPC hydrogel was used to detect Hg²⁺ in cosmetics and water samples to evaluate its practicability. The recovery for the detection of Hg²⁺ was 98.8%~102.6% and the relative standard deviation was 0.57%~1.09% for a commercially lipstick, and they were 97.2%~103.4% and 0.61%~0.99% for the tap water, respectively. The results demonstrated that our constructed smart 2DPC hydrogels have good applicability, accuracy and reliability for the detection of Hg²⁺ in real samples. The method is simple for testing, only requiring a laser pointer and a ruler, without needing sophisticated instruments, and it can realize portable detection, providing a new strategy for detecting metal ions.