Stellar parameters and magnetic activity are important for the understanding of stellar physics and dynamo theory. We have used LAMOST spectroscopic surveys to study stellar parameters and magnetic activity properties of TESS objects. There are 46,684 spectra of 31,385 TESS objects in the LAMOST low-resolution spectral survey. We have calculated equivalent widths of Ca ii H and K, and H alpha lines, and discussed the relationship of chromospheric activity with stellar temperature. There are 25,710 spectra of 10,404 stars with repeated low-resolution observations. We found that there are 694 objects with variations of the H alpha line and 1406 objects with varying radial velocities. Moreover, there are 14,099 spectra of 5834 TESS objects in the LAMOST medium-resolution survey, of which there are 2666 objects with H alpha variations and 1480 objects with variation in radial velocities. By using the light curves of the TESS objects in the LAMOST field, we detected 4269 flare events and determined their flare parameters. We have confirmed that the fraction of the flare stars increases as stellar temperature decreases. There is also an observed relationship between the flare energy and chromospheric activity. We have calculated the power-law index between the flare energy and decay time using TESS data to be 0.27(0.01), which can be explained by the magnetic reconnection theory. We confirmed that RH alpha & PRIME; R < 0.1 and decreases with the increase of Rossby number R > 0.1. There is a positive correlation between the brightness variation amplitude due to star spots from TESS and the H alpha intensity from LAMOST.