A chain saw includes a housing, a motor, a chain saw bar assembly, a tensioning assembly set on the housing, and an adjustment assembly set on the side cover corresponding to the tensioning assembly. The chain saw bar assembly is detachably connected to the first housing through the combination of the tensioning assembly and the elastic components, the adjustment assembly is movably connected with the tensioning assembly, and the adjustment assembly can adjust the relative position of the tensioning assembly and the guide shaft, to tension the chain set on the circumferential direction of the guide bar and tightly lock the chain saw bar assembly between the first housing and the side cover.