The utility model relates to a foundation pit drainage device for subway construction, which belongs to the technical field of drainage equipment and comprises a limit rod, a movable plate, a weight box and a water absorption structure, the limit rod is fixed in the middle of the top of the weight box, the movable plate is movably sleeved on the outer ring of the limit rod, and the water absorption structure is arranged at the top of the movable plate. The weight box is a hollow box body, an opening is formed in the front side of the weight box, a box door is connected to the opening of the weight box in a clamped mode, sealing rings are installed at the opening of the box door and the opening of the weight box, the box door and the weight box are arranged in a sealed mode, a balancing weight is placed in the weight box, and an annularly-arranged air bag is fixed to the bottom of the movable plate. When accumulated water exists in the foundation pit, the weight box directly sinks into the accumulated water to drain water, mounting and dismounting are not needed, time is saved, the device can be directly taken out of the foundation pit after the accumulated water is completely drained, and the follow-up construction progress in the foundation pit cannot be affected.