In China, the large-scale renewable energy bases are typically constructed in northwestern deserts and Gobi regions with altitudes exceeding 4000 m, where high flux of cosmic rays seriously threatens the safe operation of insulated gate bipolar transistors (IGBTs) in high-voltage direct current (HVDC) valves, particularly due to irradiation-induced single-event effects (SEEs). Previously, we experimentally investigated radiation resistance of trench-gate IGBTs and analyzed their irradiation responses using commonly adopted half- and single-cell TCAD simulations. However, these simulations often overestimate the electrothermal response, leading to significant discrepancies with experimental results. In this article, a theoretical analysis of the irradiation responses of trench-gate IGBTs is conducted via multicell simulations, the results of which are exactly consistent with experimental failure behaviors. It is revealed that the irradiation-induced reconstruction of electric field at the bottom of the trench-gate triggers single-event gate rupture (SEGR), which is the primary contributor to the irradiation-induced failure in trench-gate IGBTs. In addition, current commutation and local high-temperature point transfer between adjacent cells are observed during irradiation. Consequently, the highly localized transient current and high electric field induce a rapid temperature rise that may degrade the p-n junction, but is insufficient to cause single-event burnout (SEB) in trench-gate IGBTs under typical operating condition (i.e., LET = 10 MeV center dot cm(2)/mg and V-CE = 2800 V). On this basis, the influence of dummy-gate design on radiation resistance is also analyzed to facilitate the improvement of overall device performance in future designs.