This paper explores the influence of the confining stress on rock fragmentation under the cutter of tunnel boring machine (TBM) by indentation test. Acoustic emission (AE) system was employed for real-time monitoring the rock fragmentation process during the indentation test. Granite and marble samples were used in the indentation test. The rock fragmentation processes with the different confining stress levels was studied based on the characteristics of force–penetration depth curve and the AE parameters, including AE energy rate, hit counts, amplitude and location of AE events. After the indentation test, the rock samples were cut along the centerline of rock samples and subsequently the crack distribution and the size of crushed zone were measured. The test results showed that the force for crack initiation and crushed zone size increased with increasing confining stress. The cracks tend to propagate to the free surface as the confining stress increases. But for the marble samples, there exists a critical confining stress condition, such that if the confining stress is more than the critical value, the force for crack initiation and crushed zone size decrease dramatically. Moreover, it was found that rock slabbing induced by confining stress had occurred before TBM cutter indentation. Such confining stress condition promotes rock fragmentation. •Influence of confining stress on rock fragmentation by a TBM cutter is studied by indentation test.•AE technique is used for real-time monitoring rock fragmentation during indentation test.•The damage zone and crack distribution are observed by the cross-section of the failed samples.