The effects of controlling dissolved oxygen tension(DOT) on cellulase production by Trichoderma reesei were studied with 10 g/L pulp. The moore content of carbon dioxide and oxygen in off-gass and cellulase activity were mensurated. The results showed that at aeration rate of 25 L/(h～L), the DOT was too low for 22-40 h, but it was high for 0-15 h and 45 h to end. The moore content of carbon dioxide in the gas was above 0. 8% for 45-80 h, and it went against the growth of mycelium. When the aeration was adjusted to control DOT, cellulase activity was effectively increased and the time of cellulase production was evidently shorten. At DOT of 20%-30%, the content of carbon dioxide in the gas was dropped to about 0.4%, the growth and metabolize of mycelium was better and filter paper activity (FPA) reached 3.54 U/mL. It was 28.7% higher than 2. 75 U/mL of the unchanged one. It was found that the optimnm time for harvest of cellulase was shortened from 84 h to 72 h.