Deep well drilling speed reducer and working method thereof

The invention relates to a deep well drilling speed reducer comprising a stub-tooth planetary reduction gear, a pressurizer, an axial force thrust terminator and a drilling bit connecting seat. The speed reducer has the working method as follows: when the deep well drilling speed reducer arrives at a shaft bottom, both ends of all sealing devices are equal; under the action of an eccentric shaft journal, a pinion at an upper end of a duplicate planetary gear rolls in a fixed gear ring of a shell provided with the gear ring; an output shaft with a pinion only outputs rotation after speed reduction to realize gearing down and amplify the torque according to the reduction gear ratio; a thrust shaft transfers axial thrust generated during drilling to a thrust joint by a thrust bearing which can bear huge axial thrust force; the speed reducer can run smoothly by dint of the eccentric mass of a balance weight; high-pressure slurry arrives at a bit through a slurry passage to cool the bit and sweep away rock powder; because the pressurizer is adopted, the speed reducer has equal internal and external pressure, and the sealing devices are not easy to damage. The invention has the advantages that the speed reducer can be used in deep wells with the depth larger than 5,000 m, can smoothly run at the high temperature of 100-200 DEG C and the high pressure of over 50MPa, and has small volume and large transmission power.