The invention relates to a drainage system for an aluminum alloy pedestrian bridge. The drainage system comprises a first vertical drainage tube, a second vertical drainage tube, a third vertical drainage tube, a first longitudinal drainage tube and a second longitudinal drainage tube, wherein two drainage holes are formed in both sides of a bridge surface sidewalk plate above the piers of the aluminum alloy pedestrian bridge and the inner sides of guard bars; one ends of the first vertical drainage tube and the second vertical drainage tube are communicated with the drainage holes respectively; the first and second longitudinal drainage tubes are arranged in the piers along the longitudinal direction of the piers; one ends of the first and second longitudinal drainage tubes are communicated with one port of a 90-degree downstream three-way valve respectively; the other end of the first longitudinal drainage tube is communicated with the first vertical drainage tube through a 90-degree bend; the other end of the second longitudinal drainage tube is communicated with the second vertical drainage tube through a 90-degree bend; the upper end of the third vertical drainage tube is communicated with the third port of the 90-degree downstream three-way valve; and the lower end of the third vertical drainage tube is communicated with a rain well. According to the drainage system, the drainage problem of the aluminum alloy pedestrian bridge can be well solved, and the entire drainage system is attractive.