The 2.94 μm nanosecond erbium laser is an important solid-state laser source in the wide-tuning mid-infrared laser and clinical medical research. In this work, a novel LiNbO₃ acousto-optically Q-switched Er:YAG laser is developed, and the effects of different Q-switched delay times and output coupler’s reflectivities on the laser output pulse characteristics are investigated at a repetition frequency of 20 Hz. A concave-convex resonant cavity is designed to compensate for the thermal lens effect, and a single Q-switched pulse is obtained. The maximum pulse energy and minimum pulse duration are 34.68 mJ and 119.9 ns respectively, with corresponding peak power of 289.24 kW. Compared with the plane-plane cavity, the cavity proposed herein increases the output energy by 2.09 times. To our knowledge, this is the highest energy ever obtained in the acousto-optically Q-switched Er:YAG laser. This work provides a new means for further studying wide-tuning mid-infrared laser technology.