Optical fiber lasers have become a highlight in the laser field in recent years. Compared with conventional solid-state lasers, optical fiber lasers have many advantages such as easy to form single mode and good beam quality, etc. The principle of double cladding Ytterbium doped fiber and its amplification are discussed. An acousto-optic Q-switched optical fiber laser is designed and relative optical fiber components such as laser fiber isolators are not to be used. The laser has the advantages of compact structure, high efficiency and narrow spectral line width, etc.