A fixing device for a switching box of a light pipe type daylight illumination system comprises a ceiling joist made of H-shaped box iron and a U-shaped snap joint installed between the ceiling joist and the circular rotating square switching box, and the U-shaped snap joint is formed by bending a metal sheet. The U-shaped snap joint is reversely arranged in a groove between the upper horizontal plane and the lower horizontal plane of the ceiling joist, the length of the snap joint is matched with the width of the groove, and the top plane of the U-shaped snap joint and the ceiling joist are connected together through self-tapping screws. The bottoms of clamping arms are provided with a flat folding part and a bending part respectively, and the flat folding part and the bending part are used for performing fixing and connecting respectively, and the clamping arms are connected together through bolts in the middle. According to the fixing device, the circular rotating square switching box and the ceiling joist are fixed tightly, the structure is simple, cost is low, efficiency is high, operation is convenient, the problem that the switching box is not fixed easily and is prone to being infirm in fixation because of change of the shape diameters of a switching box body is solved, and meanwhile the problems that an existing fixing device is high in exclusiveness and cannot be used for fixing the circular rotation square switching box and the ceiling joist of the light pipe type daylight illumination system are solved.