High specific surface area activated carbons were prepared from coal by KOH activation in flowing nitrogen. The influences of activation temperature，impregnant ratio and activation time on the adsorption capacity were investigated. Nitrogen adsorption isotherms at -196℃，specific surface area and pore distribution of high specific surface area activated carbon were measured. The results showed that high specific surface area activated carbon with high quality could be prepared at an activation temperature of 900℃，an impregnant ratio of 4 and an activation time of 1. 5 h. The specific surface area，pore volume and adsorption iodine values of activated carbon at optimized parameters were 3135 m2·g-1，1.72 cm3·g-1 and 2657 mg·g-1，respectively. Microstructures of high specific surface area activated carbons were observed with scanning electron microscopy（SEM）. Off-gas released during the activation process was detected and analyzed by SWG300 gas analyzer. The mechanism of preparation of activated carbon from coal by KOH activation was proposed.