A hair trimming device has a first knife, a second knife, a first protective cover, a pair of movable plates, an elastic element and a second protective cover. The first protective cover covers the first knife and has a first comb installed at a front end of the first knife, and the pair of movable plates are linked with both external sides of the second knife and the first knife, and the second protective cover is installed at a front end of the pair of movable plates, and a second comb is disposed between the first comb and the cutting blade, such that the movable plate is linked to the second comb. When use, hairs of different lengths are bent and fixed and then cut to achieve layered hair styling without any discomfort caused by tearing the hair.