Q-switched ytterbium-doped double-clad fiber laser has been studied in both theory and experiment, and theory model is built. A novel method based on 'the relationship between the energy utility ratio and the initial particle inversion' to solve the final particle inversion remaining is advanced . According to the rate-equation theory of the Q-switched fiber laser, the general law that some important initial parameters, such as pump power, fiber core diameter, fiber length, output transmittance and fiber inherent loss influence the pulse width and energy is worked out. The analysis of these parameters is of important reference to designing the kindred Q-switched fiber lasers. In the experiment, a D-shape ytterbium-doped double clad gain fiber pumped by laser diode is used. A pigtailed acoustooptic Q-switch is inserted between the totally reflected fiber Bragg grating (FBG) and the combiner’s signal port. During the experiment, the operation of the acoustooptic Q-switched Yb3+-doped all-fiber laser is realized. The repetition rate can be adjusted continuously between 10 Hz and 100 kHz. The 3 μs pulse width and the 2.94 mJ pulse energy are obtained at the repetition rate of 500 Hz.