Lasing action in strongly coupled plasmonic nanocavity arrays was investigated both theoretically and experimentally. This phenomenon was generated by coupling a gain medium with a surface lattice plasmon resonance mode. In this paper, the near-field properties of this laser are investigated theoretically. It is showed that the lasing action is associated with a strong near-field enhancement at the emission wavelength that can be 4 orders of magnitude higher than what could be generated by the same nanostructure without the gain medium. The field enhancement depends non-linearly on the pumping energy. Finally, the enhancement mechanism in such an active system is assumed to rely mainly on energy transfer at the nanoscale from molecules composing the gain medium to the plasmonic mode.