Based on the optical antenna theory, the emission quenching of tris-(8-hydroxyquinoline) aluminum (Alq3) by silver nanoprisms is investigated by experiments and numerical simulations. The emission quenching is attributed to the coupling between the emission of Alq3 and the in-plan electric quadrupole resonance of silver nanoprisms. In experiments, the quenching factor is 0.25~0.5. As the density of silver nanoprisms on the substrate increases, the dimers and trimers increase. It enhances the near-field electric field, improves the excitation efficiency and decreases the quenching factor. Numerically, the quenching factor is about 0.44. It is agreement on experimental results. The study is significant to apply the nanoprism for molecular sensor and surface-plasmon-assistant image.