The mechanism of the hydrolysis of acetylcholine (a neurotransmitter) catalyzed by enzyme in Torpedo califomica was verified on structure and energy using QM/MM method. Structures of intermediates (state when substrate approaches active site, enzyme-substrate complex, and state when choline is eliminated) were optimized. The transition states were located with the estimation for the activation energy was 4.581 kcal/mol for the formation of enzyme-substrate complex, and 7.598 kcal/mol for the elimination of choline.