Provided is a fixing transmission structure for a rotary drilling frame of an ultralarge-caliber drilling rig. A thread structure and an axial sliding key groove structure are arranged on the periphery of a vertical rotary pipeline of the inverted-T-shaped rotary drilling frame from top to bottom. When a motor drives a driving pinion to rotate, a big driving gear drives the rotary pipeline to rotate in a key matching mode, and the rotary pipeline rotates through matching with a big nut. An axial sliding key of the sliding key groove structure is provided with the big driving gear in a matched mode. The thread structure is provided with the fixing and spirally-pushing big nut in a matched mode, the upper portion of the big nut is provided with an upper supporting bearing and an electromagnet, the lower portion of the big nut is provided with a middle rotating supporting bearing, the big driving gear, a lower supporting bearing and a cross-shaped supporting frame in sequence, and the big nut abuts against the outer wall of the rotary pipeline. The fixing transmission structure is one key part of the ultralarge-caliber drilling rig, can fix, rotate, feed and retreat a working machine frame of the drilling rig at any time, and is simple in structure, flexible in transmission and small in horsepower loss.