A light-up accessory which includes a plurality of elementary building units and a chaining arrangement, wherein each of the elementary building units includes a supporting case; a lighting system, which is disposed in the supporting case, and includes an illuminating unit; an operating system which includes an operating member electrically connected with the lighting system and operatively connected with an electrical power supply in such a manner that when the operating member is switched on, the illuminating unit is supplied with electrical power to illuminate. The chaining arrangement which contains a plurality of chaining passageways longitudinally formed on the supporting cases of the elementary building units respectively, and includes a connecting chain adapted to connect the elementary building units by passing through the chaining passageway of each of the elementary building units.