Two-dimensional transition metal dichalcogenides exhibit remarkable optical properties. However, their applications in electronics and photonics are severely limited by the intrinsically low absorption and emission rates. Here, the photoluminescence (PL) enhancement by integrating the monolayer MoSe2 into an Ag nanowire-on-mirror (NWoM) nanocavity is reported. From the dark-field scattering spectrum, a Fano resonance resulting from the coupling between discrete exciton state of MoSe2 and broad plasmon mode of nanocavity is observed. This Fano resonance, as a characteristic of intermediate plasmon-exciton coupling, shows remarkable ability to accelerate emission rate of MoSe2. Furthermore, the nanocavity with multiple resonances provides an excellent spatial mode overlap at excitation and emission wavelengths that affords the intriguing opportunity to resonantly enhance the absorption and PL quantum yield at the same location. The combination of Fano resonance and mode matching allows the attainment of over 1800-fold PL enhancement. These results provide a facile way to enhance the PL intensity of monolayer MoSe2 that may facilitate highly efficient optoelectronic devices.