A novel gain medium structure is designed providing Q-switched pulses with high efficiency. The use of an improved corner-side hybrid pump structure achieves a high absorption efficiency of pump light by crossing through the active media 13 times Nd:YAG. A layer of Sm:YAG is bonded around the active media, which suppresses parasitic oscillation and amplified spontaneous emission (ASE) effectively. A high-efficient actively Q-switched laser is successfully realized with a pulse energy of 104 mJ. The corresponding optical-optical conversion efficiency is 30.5%. This is, to the best of our knowledge, the highest optical to optical efficiency for an actively Q-switched Nd:YAG laser at the hundred milli-joules level.