The loose zone is of essential importance to the supporting of underground caverns. It changes dynamically with the change of time, excavation depth, and lithology of the surrounding rock. To provide reference and guidance for alike projects, the applicability of sonic wave method in dynamically analyzing loose zone is expounded, and the loose zone at upstream sidewall of an underground powerhouse is measured and tested by the method. Curves of the loose zone versus the depth of excavation, the wave velocity of rock mass as well as the time and space are given. By analyzing the curves, the variation and distribution of loose zone are obtained, and corresponding engineering measures for the project construction are put forward.