In this paper, the causes for the failure of the main drive reducer of shield machines are studied in terms of 〖HJ2mm〗synchronism of electric motor, disassembly of damaged reducer, break of safety axis and boring parameters of shield machine, with the shield machine for the 500 kV power tunnel in Haidian District in Beijing as an example.The main causes for the failure of the main drive reducer of the shield machine are as follows: 1) Irrational boring parameters; 2) Failure of rolling bearing of the 2nd grade and the tertiary reducers.The paper can provide reference for shield boring and reducer failure treatment in the future.