The mechanisms and kinetics of dissolution of mechanical activated light-calicned magnesite were investigated at the temperature range of 30℃ to 70℃,(NH4)2SO4 from 0.05 M to 0.3 M, and dissolution time from 0 to 300 min as a function of grinding time.The experiments show the apparent activation energy is significantly reduced with the increasing of grinding time, and the order of apparent reaction seems to be increased correspondingly.As a result, the mechanical activated method of light-calicned magnesite has a good application prospect in the refinery chemicals of magnesite.