For the reasons that cause the inner wall of the mouth to grow black spots, specifically as follows: 1, the upper and lower back teeth for shallow overlay jaw, shallow cover or to the edge of the jaw: will often bite the lip and cheek mucosa, resulting in ulceration, producing bruises, black spots. 2, the formation of residual roots and crowns in the posterior region: its sharp edges stimulate the buccal side of the mucosa to produce fibrous hyperplasia, and the texture of black. 3, long-term smoking: tobacco burning, so that the gums, oral mucosa appears dark gray, brown-black 4, the presence of heavy metals in the body: such as lead, bismuth, etc., the lip and cheek mucosa can form black lines, black spots. 5, the application of silver amalgam filling: silver particles can be deposited on the surface of the mucosa, and gray-black spots. 6, suffering from systemic diseases: such as nephrotic syndrome, etc., the oral mucosa can appear gray-black, dark brown spots.