The utility model discloses a tubular PTC constant-current electrical heating tube for heating liquid. A PTC ceramic combination includes a PTC ceramic body, and an upper electrode plate and a lower electrode plate disposed on and under the PTC ceramic body respectively. The PTC ceramic combination is disposed in a metal tube through an insulating layer. The shape of the combination of the PTC ceramic body, the upper electrode plate and the lower electrode plate is a circle. According to the tubular PTC constant-current electrical heating device, because each of the metal tube and the combination of the PTC ceramic body, the upper electrode plate and the lower electrode plate is in the shape of a circle, the heat conduction becomes uniform; insulators disposed at two ends of the metal tube help to prolong the service life of a product; the electrode plates with the width identical to the PTC ceramic body are adopted and edges of the electrode plates exceed the edge of an electrode on the surface fo the PTC ceramic body, so that the generation of an electric arc is prevented, the insulation reliability is substantially improved, and the service life of the electrical heating tube is prolonged; the maximum movable gap width of the upper electrode sheet or the lower electrode sheet from the left inner side or the right inner side of the metal tube respectively is smaller than the electrode margin width on the PTC ceramic body, and the free swinging of the PTC ceramic combination in the metal tube does not expose the electrode to the outside.