A high power actively Q-switched mid-infrared fiber laser at 2.8-μm waveband is demonstrated by employing a 975 nm laser- diode pumping a piece of heavily Er3 +- doped ZBLAN doubleclad fiber and a mechanical chopper into the cavity as the Q- switch. Output characteristics at different pump powers and various repetition rates is investigated. Meanwhile, the pulse spikes are observed and analyzed. The maximum pulse energy of 84.5 μJ, pulse duration of 250.3 ns and peak power of exceeding 300 W is obtained at a repetition rate of 3.5 kHz under a proper pump power.