A label-free fluorescent DNA-based sensor for the detection of Pb²⁺ and Hg²⁺ has been reported in this paper. A single-stranded DNA named modified T30695 was used as a recognition probe and SYBR Green I (SG) was used as a signal reporter. This sensor consisted of two interaction sections: Pb²⁺ interacts with modified T30695 to form a G-quadruplex and Hg²⁺ interacts with modified T30695 to form a T–Hg(II)–T hairpin conformation. Circular dichroism confirmed the interactions between modified T30695 and Pb²⁺ or Hg²⁺. Based on this, a sensor for the detection of Pb²⁺ and Hg²⁺ with limits of detection of 2.09 ppb and 1.14 ppb was constructed.