We have used the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) low-resolution and medium-resolution spectroscopic surveys to explore the chromospheric statistical properties and variations of M stars. There are 738477 LAMOST low-resolution spectra of 622523 M stars. We have calculated equivalent widths of the CaII H&K, H{alpha}, H{beta}, H{gamma}, H{delta}, HeI D3, and CaII infrared triplet lines. Based on the H{alpha} EWs, we found that 33374 objects showed H{alpha} emissions. We revised the relationship between the active fraction and spectral subtype, and confirmed that the active fractions increased from M0 to M3, and possibly declined from M4 to subsequent types. Among 86005 objects with repeated spectra, 19422 objects showed H{alpha} variations. The variation fraction of stars with M4 and subsequent types with a fully convective envelope was higher than those of M0-M3 with a partially convective envelope. By combining the distances from Gaia, we have determined the positions of the M dwarfs in the catalog for M stars. The active fraction decreased rapidly in the height range of 0-400pc above and below the Galactic plane, and maintained a steady trend in 400-1000pc. We cross-matched the M catalog of the LAMOST low-resolution survey with LAMOST medium-resolution spectra. We obtained 272181 spectra of 13355 M objects and calculated the H{alpha} EWs. Among them, 3813 stars show variations, which might be caused by chromospheric activity evolution with orbital phase or time. The H{alpha} emission for 972 objects shows variations over short and long timescales, especially on short timescales of 20 minutes.

Cone search capability for table J/ApJS/253/19/mstars (Parameters of M stars catalog observed in LAMOST low-resolution spectral survey DR7 (Tables 2 and 3))

Cone search capability for table J/ApJS/253/19/table5 (Parameters of M stars catalog from LAMOST medium-resolution survey, the parameter of H{alpha} line and its variations in short and long time scales)

Cone search capability for table J/ApJS/253/19/table6 (Parameters of M stars catalog with repeated LAMOST low-resolution observations)