The cubic mudstone specimens are indented with constant cross-section(CCS) disc cutter to study the fragmentation characteristics of soft rock in indentation tests with or without lateral confinement. The acoustic emission technique and scanning electron microscope analysis are employed to analyze the failure information. The load-penetration curves and acoustic emission parameters are studied with the observation and detection of macrocrack formation processes and mesoscopic failure characteristics. The results show that: (1) The load-penetration curves appear distinct peak value in both cases, after that the unconfined specimen is completely broken down and the confined one still could bear load at a pretty high level. (2) The acoustic emission parameters reach the peak values when the main macroscopic crack forms and propagates in the unconfined test; the acoustic emission parameters arise for several times in accordance with the initiation and propagation of certain number of macrocrack in the confined test. (3) Micro-slips within mudstone specimens can act as evident shear cracks; and the mudstone fragmentation pattern in indentation test with CCS disc cutter is mainly controlled by shear failure.