The invention relates to a flue structure, in particular to a large space frame type gas collection lower flue structure. The large space frame type gas collection flue structure divides a gas collection lower flue into 2-10 segments, each segment of the flue is provided with separate branch smoke pipes for collecting gas separately, the gas is gathered into a main smoke chamber, the main smoke chamber is communicated with a main smoke exhaust pipe, the gas collection lower flue is arranged at the bottom of a horizontal cover plate, each branch smoke pipe is welded to the upper portion of thehorizontal cover plate, and the bottoms of girders are welded to the tops of the branch smoke pipes. According to the structure, a gas collection effect of the flue is improved, the contact area between the girders and the horizontal cover plate is increased, the strength of the girders is improved, the height of the girders can be effectively decreased, the height of an electrolysis plant is decreased at the same time, and the civil construction investment is saved.