Stellar magnetic activity is an interesting phenomenon in late-type stars. We use the medium-resolution spectroscopic observations of 406069 late-type stars from LAMOST to explore their properties. We perform a statistical analysis on the magnetic activity of the stars and their associated physical parameters. Our samples are cross-matched with other data bases (LAMOST DR5, VSX, and SDSS DR12) to obtain additional observational properties. The equivalent widths (EWs) of H{alpha} lines, an indicator of chromospheric activity, are calculated. According to the EWs of the H{alpha} line, we detect 8816 spectra with apparent H{alpha} emission for a total of 2115 stars among the 2108565 spectra analysed. Furthermore, 1521 of these stars show variability in their H{alpha} lines. In addition, we detect 2132 flare events associated with 41542 stars from the catalogue by cross-matching our LAMOST medium-resolution samples and the Kepler and K2 data bases. We also confirm a weak relationship of the flare amplitude with increasing Rossby number. There is a clear decease in the strength of chromospheric activity (L_H{alpha}_/L_bol_) with increasing rotation period. Moreover, the ratio of the flare energy and stellar luminosity is found to decrease with the stellar mass.