The invention relates to a drainage device for a steel truss girder box-section member. Drainage holes are reserved on a bottom plate and close to a member seal-end partition in the range of a splice joint of the steel truss girder box-section member, and drainage steel pipes are additionally welded at the outer edge of the bottom plate of the girder box-section member and below the drainage holes. The drainage steel pipes are additionally welded at the outer edge of the bottom plate of the member and below the drainage holes which are reserved the bottom plate and close to the member seal-end partition in the range of the splice joint of the steel truss girder box-section member, so that a collected water guide device is formed, collected water in the splice joint of the box-section member is drained out of the member joint and drained out of the drainage steel pipes, the foul water cannot flow everywhere, late maintenance difficulty and risk of a main structure of a steel truss girder can be reduced, and attractive appearance of the girder can be maintained. The drainage device is simple in structure, convenient and quick in manufacture and mounting and excellent in economic performance, has bright application prospect and can also be used for bolted steel box girders.