The invention discloses an overflow depth adjusting device in a horizontal spiral sedimentation centrifuge. The overflow depth adjusting device comprises multiple overflow parts arranged on the bottomwall of the liquid phase discharging end of a drum. An overflow opening is formed in each overflow part. The overflow depth between each overflow opening and the inner side wall of the drum can be adjusted by rotating the corresponding overflow part. A rotatable outer gear ring is arranged on the outer side of the liquid phase discharging end of the drum. The overflow parts are located on the inner side of the outer gear ring. Small gears engaged with the outer gear ring are fixedly arranged at the outer ends of the overflow parts. The overflow depth adjusting device is further provided witha locking mechanism capable of locking the outer gear ring and/or the small gears with the drum. According to the overflow depth adjusting device, the overflow depths of the overflow openings can be adjusted at the same time by rotating the outer gear ring, after adjusting, the overflow depths can be locked at the same time by locking the outer gear ring and the drum, and therefore adjusting operation of the overflow depths becomes simple and rapid, and adjusting efficiency is greatly improved.