Electrochemical detection of myricetin is rapid and economic but has been rarely reported so far. Here we present a layer-by-layer self-assembly strategy to fabricate a ternary nanocomposite film containing polyoxometalate P2W18 clusters, mesoporous SnO2 nanospheres and Au nanoparticles (AuNPs) on the ITO electrode for the electrochemical determination of myricetin. Compared with all the other electrodes herein and previously reported, the (P2W18-SnO2-AuNPs)3/ITO widened the electrochemical determination range of myricetin from 1 to 110 μmol/L with quick response time, which was extended for the myricetin detection in juice samples with satisfactory recoveries between 97.65% and 103.07%. The extraordinary electrocatalytic activity could be attributed to the synergetic contribution of three components, namely P2W18 with multi-electron reversible electrocatalytic properties, mesoporous SnO2 nanospheres with high conductivity and electroactive surface area, and AuNPs with outstanding conductivity and electrocatalytic properties. With superior stability, reproducibility and anti-interferences against common coexisting ions, the presented method would cater for the requirements of clinics and pharmaceuticals for the rapid and sensitive determination of myricetin in real samples.