The invention discloses a draining device for building bridges. The draining device comprises a steel structural rack, a concrete beam, a waterproof layer and an asphalt pavement, wherein the concrete beam is positioned at the upper part of the steel structural rack; the waterproof layer is paved on the upper part of the concrete beam; the asphalt pavement is paved on the upper part of the waterproof layer; a draining groove is formed in the asphalt pavement; a draining groove cover plate covers on the draining groove; a draining pipe is embedded into the asphalt pavement, and a group of water inlet pipe nozzles are externally connected to the position, at the lower part of the draining groove, of the asphalt pavement; an overflow pipe is externally connected to one side of the draining pipe; one end of the draining pipe is connected with a water storage tank; an air compressor is detachably connected with the top of the water storage tank, and a feed pipe is externally connected with the bottom of the water storage tank; an electronic valve is arranged on the draining pipe in a connecting manner. The draining device has excellent waterproof performance, good draining effect and glide prevention property, can be used for collecting water and watering a bridge surface for cooling in hot weather in summer, and is humanized in design.