The invention mainly discloses an illuminating system for a historic building gallery bridge and a construction method thereof. The illuminating system comprises a roof, ceiling lamps and stand columns; the stand columns are symmetrically supported below the roof; the ceiling lamps are connected to the stand columns through arc hollow pipes; each ceiling lamp comprises a lamp base, a lamp frame and a lamp cap; a wire winding block, a wire groove and a battery base are arranged in each lamp base; the arc hollow pipes penetrate through the lamp bases, and are provided with wire penetrating holes in the lamp bases; each lamp frame consists of a first connecting shaft, a baseplate and vertical plates, wherein the vertical plates are separately fixed on the left and right sides of the baseplate, the baseplate is connected with the corresponding lamp base through a second connecting shaft, two ends of the first connecting shaft are connected to the vertical plates in a rotating manner; and each lamp cap is connected to the corresponding first connecting shaft. The illuminating system is simple in structure, convenient to install and adjust and low in cost, saves the space, cannot destruct the internal structure and the appearance design of a building, is low in influence on the gallery bridge, and not only can guarantee integrity of a historic building without damaging, but also can achieve a better illuminating effect.