Abstract: It presented the acoustic property of double-decked duct in Multi-functional Theater. It analyzed the muffling function of double-decked duct to find out its role in the indoor acoustic comfort, using the indoor noise measurement data under different loads of air conditioning operation. It showed the acoustic design was satisfied by measurement result. Noise levels in each measurement points and times were different, but not significantly. As operating load enlarged, the net increase of noise was small. Then the muffling of double-decked duct ensured the acoustic comfort of the theatre.