Large Purcell enhancement, requiring high-quality factors and small mode volumes, is essential to single-photon sources. Whispering gallery microcavities possessing high-quality factor are limited by large mode volume, while dielectric nanoantennas with ultra-small mode volume suffer from significant scattering loss. Here, by combining the advantages of the microtoroids and the nanoantennas, we achieve large Purcell enhancement with narrow linewidth in all-dielectric nanoantenna-microtoroid hybrid structures. The scattering loss of the nanoantenna is suppressed by the high-Q microtoroids, meanwhile its ultra-small mode volume remains almost unchanged. As a result, the Purcell factor of the emitter located at the gap of the nanoantenna reaches as high as 1000 \textasciitilde 1700, while its linewidth is kept at the order of hundreds of picometers. The proposed mechanism holds promise for applications in on-chip single-photon sources and low-threshold nano-lasers.