The invention discloses a bridge externally hung drainage channel joint device and a drainage system. A bridge comprises multiple sections of beam bodies; a section of drainage channel is mounted on each section of beam body in an externally hung manner; the bridge externally hung drainage channel joint device comprises a telescopic pipe and an end sealing plate; a circular steel pipe is mounted at each of two ends of each section of the drainage channel and stretches outwards along the drainage channel, and the external diameter of the circular steel pipe is less than the width of each section of drainage channel; one end of the circular steel pipe fits the end face of each section of the drainage channel; the end sealing plates seals the space between the outer wall of the circular steelpipe and the inner wall of the drainage channel; the telescopic pipe is mounted between two adjacent sections of water channels; and the two ends of the telescopic pipe are fixedly arranged on the circular steel pipe in a sleeving manner.