An experimental study of Yb-doped double-clad fiber laser in various optical configurations operating in self-Q-switched, AOM Q-switched and hybrid Q-switched conditions is presented. The fiber with a D-shaped inner cladding is pumped by a multi-mode laser diode with high power at 976 nm. Three thresholds of the Q-switched fiber laser are observed and analyzed. And the characteristics of the output pulses are studied experimentally in detail. Pulses with duration of ns, peak power of about 200 kW and the repetition rate of kHz are obtained. The results of the experiment are explained qualitatively.