A 3 μm passively Q-switched Ho³⁺/Pr³⁺ co-doped fiber laser by using Au nanocages as saturable absorber is demonstrated. Stable Q-switched pulse trains with a repetition rate of 82.0 kHz are initially obtained as the pump power increases to 99.7 mW. When the pump power increases to 347.1 mW, the largest output power of 50.7 mW with a pulse duration of 2.21 μs and a corresponding repetition rate of 169.5 kHz is obtained. This study indicates that Au nanocages have a great potential as an outstanding optical modulator for mid-infrared pulses generation.