Scars are the result of abnormal repair of skin tissue trauma. Recently, fractional laser is more and more widely used in the treatment of scars, but its mechanism is not clear. Studies have shown that fractional laser could produce multiple microthermal zones in target skin, induce wound repair responses, affect the function of epidermal and dermal cells, induce changes in blood vessels and collagens, and change the expression of heat shock proteins, microRNA, matrix metalloproteinases, cytokines such as transforming growth factor β, basic fibroblast growth factor, and facilitate drug delivery, thus achieving the effect of treating scars. This article reviews the mechanism of fractional laser in treating scars from three aspects, including the tissue and cell mechanism, molecular mechanism, and drug delivery.