This dataset of H-alpha activity indices is for investigation of stellar H-alpha chromospheric activity based on the LAMOST Medium-Resolution Spectroscopic Survey (MRS). High signal-to-noise ratio MRS coadded spectra of F-, G-, and K-type stars (4000-7000 K) from LAMOST Data Release 8 are utilized in the dataset (329,294 spectra in total). The H-alpha activity index (I_halpha) and the H-alpha R-index (R_halpha) are evaluated for the MRS spectra. The values of the H-alpha indices and their uncertainties, as well as the related spectroscopic parameters from the LAMOST catalogs, are stored in a CSV-format file (file name: Halpha_activity_indices_LAMOST_MRS_DR8.csv). Columns included in the dataset: obsid - unique observation identifier of LAMOST MRS ra - right ascension of the observed object (unit: degree) dec - declination of the observed object (unit: degree) obsdate - UTC date of the observation (format: YYYY-MM-DD) fitsname - MRS FITS data file name sn_B - signal-to-noise ratio of the blue band of MRS sn_R - signal-to-noise ratio of the red band of MRS band_para - indicating which band of MRS is used by the LASP to determine the stellar atmospheric parameters; ‘B’ representing the blue band and ‘R’ representing the red band teff - effective temperature determined by the LASP (unit: K) teff_err - uncertainty of teff (unit: K) logg - surface gravity determined by the LASP (unit: dex) logg_err - uncertainty of logg (unit: dex) feh - metallicity determined by the LASP (unit: dex) feh_err - uncertainty of feh (unit: dex) rv_r0 - radial velocity determined based on the red band data of MRS (unit: km/s) rv_r0_err - uncertainty of rv_r0 (unit: km/s) category* - ‘main-sequence’ or ‘giant’ or ‘intermediate-zone’ I_halpha* - H-alpha activity index I_halpha_err* - uncertainty of I_halpha chi* - χ factor for calculating R_halpha from I_halpha (unit: 1/Å) chi_err* - uncertainty of chi (unit: 1/Å) R_halpha* - H-alpha R-index R_halpha_err* - uncertainty of R_halpha (Note: The columns marked with an asterisk are provided by this dataset. Other columns are from the catalogs of LAMOST DR8 v1.1. LASP is an acronym for the LAMOST Stellar Parameter Pipeline. A value of -9999.0 in the dataset indicates that it is not available.)