Pen cap

Disclosed is a pen cap. A pen cap body is of a cuboid structure, a cylindrical groove is upwardly arranged at the bottom end of the pen cap body, a sponge cushion is arranged at the top end of the groove, an elastic sleeve is arranged on the lateral wall of the groove, a folding ruler movably connected to the outer wall of the pen cap consists of a plurality of elongated plate rotary shaft sections by connection, length graduations are arranged on the surface of the folding ruler, a transparent semi-cylinder structure is arranged at the upper end of the cuboid structure, an illuminating circuit arranged inside the transparent semi-cylinder structure consists of a small lamp bulb, a battery and a switch, and the tail end of the folding ruler is connected with the outer wall of the pen cap in a fastening manner. The pen cap can adapt to the sizes of various pens, can be used as a length measuring tool, can function in illuminating a road in front by connecting the illuminating circuit in dim light at night, is