The invention discloses a piston which is used in a natural gas engine and is provided with a combustion chamber, in order to overcome the problem that the prior art is low in combustion heat efficiency and combustion velocity. The invention adopts a proposal that the top surface of the piston is provided with an inward-concave crisscross combustion chamber of which an inner contour line of the transverse section is a crisscross curve; symmetry lines of the combustion chamber and the piston are collinear; the crisscross curve comprises four circular arcs with the diameter between 65 and 67 millimeters, as well as four circular arcs which form ribs of the combustion chamber and have the radius between 5 and 7 millimeters; the circular arcs with the radius between 5 and 7 millimeters are respectively placed between every two circular arcs with the diameter between 65 and 67 millimeters; the circle centers of the circular arcs with the radius between 5 and 7 millimeters are positioned outside the circular arcs with the diameter between 65 and 67 millimeters; the circle centers of the four circular arcs with the diameter between 65 and 67 mm are concentric; the circular arcs with the diameter between 65 and 67 millimeters and the circular arcs with the radius between 5 and 7 millimeters are connected by use of circular arcs with the radius of 5 millimeters; and the depth of the combustion chamber is between 25 and 27 millimeters, and the annular curved surface with the radius between 6 and 8 millimeters is used for connecting the bottom if the combustion chamber.