The invention provides an automatic locking environment-friendly chalk. The chalk is structurally composed of a chalk core and a chalk rod; the center of the chalk rod is in a hollow pipe shape, the chalk core is inserted in the chalk rod, a cap cover is arranged at the top end of the chalk rod, a clamping jaw is arranged at the lower end of the chalk rod, tooth grains are arranged on the inner wall of the clamping jaw, and the lower end of the outer wall of the clamping jaw is in a conical shape; a locking sleeve is arranged on the outer portion of the clamping jaw, the inner wall of the lower end of the locking sleeve is in an inverted conical shape, the inverted conical shape of the inner wall of the lower end of the locking sleeve is matched with the conical shape of the outer wall of the lower end of the clamping jaw, and an annular cavity is arranged between the inner wall of the locking sleeve and the outer wall of the clamping jaw; a spring is arranged in the annular cavity, and annular reinforcing ribs are machined on the outer circle of the locking sleeve; the chalk core is a cylinder with the equal diameter and length, and the chalk core is in sliding connection with the center cavity of the chalk rod; through protection of the chalk rod, force exertion points of fingers are concentrated on the tip of the chalk core, and the phenomenon that due to too large finger force, the chalk core is broken off is avoided; and the utilization rate of the chalk core is improved, generation of chalk points is reduced, environment is protected, pollutions are reduced, and the aims of being clean, sanitary and easy to carry are achieved.