Herein, Pt@tetraphenyl-1,3-butadiene nanocrystals (Pt@TPB NCs) with high electrochemiluminescence (ECL) efficiencies as ECL emitters were developed to construct an ultrasensitive biosensing platform for the detection of microRNA-21 (miRNA-21). Interestingly, Pt@TPB NCs not only exhibited high carrier densities and electron mobilities to achieve efficient electron-hole pair recombinations for high ECL emission but also served as coreaction accelerators of endogenous coreactant-dissolved O with good electrocatalytic activities to produce abundant reactive oxygen species (ROS) for facilitating the interactions between TPB NCs and ROS, which further obtain intense ECL emission. Impressively, Pt@TPB NCs with dissolved O as coreactants displayed high ECL efficiencies (Φ) of 7.83, taking the Φ of Ru(bpy)/dissolved O ECL system as 1. Herein, Pt@TPB NCs with strong ECL signals were used as ECL emitters to combine target-induced DNA walker amplification with high conversion efficiency for the construction of an ultrasensitive ECL biosening platform which accomplished microRNA-21 detection with a low detection limit of 83.8 aM. Therefore, the developed synergy effects in Pt@TPB NCs are expected to guide the progress of highly efficient ECL emitters for sensing analysis and disease diagnosis.