  The behavior of the shock wave in the atmosphere of the non-fundamental mode
RR Lyrae pulsator remains a mystery. In this work, we firstly report a
blueshifted Mg triplet emission in continuous spectroscopic observations for a
non-Blazhko RRc pulsator (Catalina-1104058050978) with LAMOST medium resolution
spectra. We analyse the photometric observations from Catalina Sky Survey of
this RRc pulsator with pre-whitening sequence method and provide the ephemeris
and phases. An additional frequency signal with $P_1/P_x = 0.69841$ is detected
and discussed. The redshift and radial velocity of the spectra are provided by
fitting process with $S\acute{e}rsic$ functions and cross-correlation method.
Moreover, we plot the variation of H$\alpha$ and Mg lines in a system comoving
with the pulsation. Clear evolution of comoving blueshifted hydrogen and Mg
emission is observed, which further confirms the existence of shock waves in
RRc pulsators. The shock-triggered emission lasts over $15\%$ of the pulsation
cycle, which is much longer than the previous observations.
