Isolation of a new structural acousto-optic switch based on an integrated optical polarization-independent quasi-collinear acousto-optic tunable filter (AOTF) is studied in detail. First, we demonstrate the structure and operation principle of this optical switch. Second, factors that influence isolation of the optical switch are analyzed, and expressions of the isolation are deduced. Finally, the optical switch is fabricated, and its isolation is measured in experiment. We show that the isolation mainly depends on the TE/TM mode intensity ratio when the radiofrequency signal is absent, but the isolation mainly depends on the mode conversion efficiency when the radiofrequency signal is present.