The volume of supply air of an underground water powerhouse is very large. Three methods of air distribution that do not occupy a lot of useful space, these methods are studied by scale model tests in this paper. The results of the tests show that the thermal environment quality at the far end away from the gate is poor if air is supplied from the gate. The velocity may be too high at some points in occupied zone if air is supplied from an end wall. This problem can be solved by reducing the supply air velocity or using an auxiliary slot. The effect of supplying air from the ceiling is satisfactory. The arrangement of one inlet under the ceiling for each generator's zone is recommended.