A small-sized hair trimming device comprising a fixed trimming block and a movable trimming block, both longitudinally, linearly and alternately arranged with multiple openings and teeth; the movable trimming block is sleeved within the fixed trimming block and movable in reciprocate movement with respect to the fixed trimming block; inner portions of the fixed trimming block teeth correspond to outer portions of the movable trimming block teeth in terms of shape and dimension, and gap does not exist in between the fixed trimming block and the movable trimming block when they meet with each other, thereby forming a trimming surface for trimming hair; the said trimming surface is concealed between the fixed trimming block and the movable trimming block without exposing to exterior environment; the small-sized hair trimming device is safe and convenient to use, and it has a reasonable structure and a bigger trimming surface.