Electrical insulation capability is one of the most critical challenges for press-pack insulated gate bipolar transistors (IGBTs), but the knowledge of its insulation failure mechanism is much less mature. To understand the insulation failure mechanism, partial discharge (PD) measurement and analysis in press-pack IGBT (PPI) submodule were performed in this study. A dedicated PD measurement system for PPIs was designed and fabricated, and fast oscilloscope, PD detector and ultraviolet (UV) image camera were employed to observe the PD. PD-induced insulation failure phenomenon in the submodule was observed under direct current voltage, and UV images indicated that the discharge occurred at the periphery of the die. Phase-resolved PD (PRPD) analysis method for PPIs was developed and three elementary structures were used to simulate the PD in the electric field reinforcement areas identified by electric field analysis. According to PRPD patterns, the observed PD in the submodule was considered to be mainly caused by a mismatch between the electric field and critical electric field on the surface of passivation layers of the die. The inference was verified at different gas pressures because a decrease of PD inception voltage in the submodule and the bare die was observed at lower gas pressure.