Abstract: The base bearing of TBM is usually of a three row cylindrical roller structure, the material of the base bearing inner ring is 42CrMo, as the gear ring that bears the transmission torque, usually has a torque of over 8 000 kN·m. During the excavation process, it can withstand alternating loads for a long time and is prone to cracking. In order to explore the mechanism of cracking on the base bearing gear ring of TBM, firstly, a non-destructive testing and analysis was conducted on a Φ3 061 mm base bearing gear ring of TBM project in South China, and it was found that there were multiple dedendum cracks that tended to be evenly distributed on the gear ring. Secondly, morphology analysis was conducted on the fracture surface of the gear ring cracking, and the brittle intergranular characteristics at the crack source indicated insufficient material toughness. Furthermore, after conducting residual stress analysis and working stress verification on the dedendum of the gear ring, it was found that the residual tensile stress was as high as 400 MPa, which exceeded the yield strength of the material after being combined with external forces. Finally, material analysis of the gear ring revealed that the content of nickel element in the matrix was relatively low at 0.13%, which reduced the material toughness and impact strength. Pearlite segregation bands were found in the matrix structure, resulting in uneven structure. The non-metallic inclusion level reached level 1.5, and higher inclusion levels increased the risk of cracking. Therefore, in order to reduce the possibility of cracks and extend the service life of the gear ring, it was necessary to optimize the gear induction quenching process and reduce the residual stress at the dedendum: increasing the content of nickel element in the matrix material of the gear ring to improve the toughness of the material; optimizing the heat treatment process of raw materials to improve the uniformity of the matrix structure; improving the purity of raw materials to reduce the level of non-metallic inclusions.