LAMOST DR8 MRS CSN Stellar Parameters and Chemical Abundances Catalog provides a catalog of stellar parameters and chemical elements (C, Na, Mg, Si, Ca, Ti, Cr, Mn, Co, Ni, and Cu) of 1.38 million LAMOST-II DR8 MRS spectra. It is the first one based on the ab initio atmospheric model and the first application of Cycle-StarNet to more than 1 million real data sets. The method incorporates the physical model and data-driven approaches to improve the shortcomings of the low S/N of observed spectra and imperfections of theoretical synthetic spectra in the fitting process. The catalog contains information: the identifier for the corresponding star (starid), LAMOST spectrum identifier (medid), Gaia identifier for 69 percent of spectra (Gaia source id), coordinate information (right ascension (RA), declination (Dec)), signal-to-noise of the spectra (S/N of blue and red arm), effective temperature (Teff), surface gravity (log g), metallicity [Fe/H], macroturbulence, microturbulence, elemental abundance ([X/Fe]), their errors and quality flags, and the total quality flag.