A screw drilling tool transmission shaft fall-proof device relates to an underground screw drilling tool, which is provided to conquer the shortage that the large end of the transmission shaft in the prior art is broken to induce malignant falling accident. The utility model comprises a transmission shaft (1), a lower radial bearing dynamic sleeve (2) and a lower radial bearing static sleeve (3), wherein the upper end of the lower radial bearing dynamic sleeve (2) is fixedly connected with the transmission shaft (1) through screw thread (5), one end of a buffer ring (4) is fixedly connected with the large head of the transmission shaft (1), and the other end is fixedly connected with the lower end of the lower radial bearing dynamic sleeve (2); the inner surface of the buffer ring (4) is in a ladder shape, and the two ends are provided with inner screw thread; and the inner screw thread on the lower end of the buffer ring (4) is connected with the upper part of the large head of the transmission shaft (1), and the inner screw thread on the upper end of the buffer ring (4) is connected with the lower end of the lower radial bearing dynamic sleeve (2). The utility model has the beneficial effect that when the large end of the transmission shaft is broken, the utility model can effectively prevent the malignant falling accident of the transmission shaft from happening.