Programming ultrasensitive and stimuli-responsive DNAzyme-based probes holds great potential for on-demand biomarker detection. Here, an optically triggered DNAzyme platform was reported for on-demand activation-sensitive electrochemiluminescence (ECL) c-myc mRNA analysis. In this design, the sensing and recognition function of the split DNAzyme (SDz) probe was silent by engineering a blocking sequence containing a photocleavable linker (PC-linker) group at a defined site that could be indirectly cleaved by 302 nm ultraviolet (UV) light. When the SDz probes were assembled on the Au nanoparticles and potassium (K) element doped graphitic carbon nitride nanosheet (K-doped g-C3N4) covered electrode, UV light activation induces the configurational switching and consequently the formation of an active DNAzyme probe with the help of target c-myc mRNA, allowing the cleavage of the substrate strand by magnesium ions (Mg2+). Thus, the release of a ferrocene (Fc)-labeled DNAzyme 2 strand contributed to an extreme ECL signal recovery. In the meantime, the released target c-myc mRNA combined another inactive SDz motif to form active DNAzyme and repeat the cyclic cleavage reaction, resulting in the signal amplification. Furthermore, according to the responses toward two other designed nPC-SDz and m-SDz probes, we demonstrated that controlled UV light mediated photoactivation of the DNAzyme biosensor "on demand" effectively constrained the ECL signal to the mRNA of interest. Moreover, false positive signals could also be avoided due to such a photoactivation design with UV light. Therefore, this study provided a simple methodology that may be broadly applicable for investigating the mRNA-associated physiological events that were difficult to access using traditional DNAzyme probes.