Using a time-dependent one-zone leptonic model that incorporates both shock acceleration and stochastic acceleration processes, we investigate the formation of the narrow spectral feature at similar to 3 TeV of Mrk 501 which was observed during the X-ray and TeV flaring activity in 2014 July. It is found that the broad-band spectral energy distribution (SED) can be well interpreted as the synchrotron and synchrotron-self-Compton emission from the electron energy distribution (EED) that is composed by a power-law (PL) branch and a pileup branch. The PL branch produces synchrotron photons which are scattered by the electrons of the pileup branch via inverse-Compton scattering and form the narrow spectral feature observed at the TeV energies. The RED is produced by two injection episodes, and the pileup branch in RED is caused by shock acceleration rather than stochastic acceleration.