Novel lanthanum ion-doped cadmium sulfide quantum dots (CdS:La QDs) were synthesized and characterized by transmission electron microscopy (TEM) and photoluminescence (PL). Based on CdS:La QDs as the electrochemiluminescence (ECL) luminophores, a distance-dependent ECL intensity enhanced or quenched system between CdS:La QDs and gold nanoparticles (Au NPs) was designed. Firstly, ssDNA 1 was linked to the CdS:La QDs modified glassy carbon electrode via amide bond. Then the prepared Au NP-ssDNA 2 conjugates were used to hybridize with ssDNA 1, the surface plasmon resonances (SPR) of Au NPs enhanced ECL intensity (signal on) while Au NPs and CdS:La QDs were separated at a certain distance. Secondly, In the presence of Hg, the oligonucleotide conformation changed from linear chain to hairpin due to the thymine-Hg-thymine (T-Hg-T) base pairs. ECL quenching (signal off) achieved lie in resonance energy transfer (RET) between the CdS:La QDs and the proximal Au NPs at a close distance. Finally, after being incubated with TB, a strong and stable TB-aptamer complex was generated, which led to the release of Au NP-ssDNA 2 conjugates. The ECL signal of the CdS:La QDs was ultimately recovered (signal on again). The "on-off-on" approach was used to detect Hg and TB, sensitively and respectively. The line ranges were 1.00 × 10 -1.00 × 10 mol L and 1.00 × 10 -1.00 × 10 mol L, respectively. The low limits of detection (S/N = 3) were at 3.00 × 10 mol L and 3.00 × 10 mol L. Moreover, the ECL sensor exhibited high selectivity and good stability, and was successfully applied to the detection of TB in real sample.