Integrated Gate-Commutated Thyristors (IGCTs) combine the low on-state voltage drop of thyristor-type devices with a wide safe operating area (SOA) enabled by hard-drive technology. With the growing adoption of IGCTs in high-voltage direct current (HVDC) transmission systems, they are progressively replacing conventional phase-control thyristors and high-voltage IGBT modules, offering substantial application prospects and market potential. This paper takes a 6500V/8000A asymmetric IGCT as the research object and systematically investigates the triggering mechanism and quantitative characterization method of the dynamic avalanche effect. Based on Poisson’s equation and the carrier continuity equations, the Fulop criterion for static avalanche breakdown is extended to transient turn-off conditions of the IGCTs. A factor g is introduced to characterize the essential difference between dynamic and static avalanche in terms of triggering conditions. Accordingly, a one-dimensional analytical physical model is established, from which explicit expressions for the dynamic avalanche triggering voltage and the critical breakdown electric field are derived. The model is validated by TCAD device simulations and double-pulse experimental tests. The model ultimately expresses the dynamic avalanche triggering voltage as an explicit function of the parasitic pnp transistor current gain α_pnp, the turn-off current density J, and the n- base region doping concentration N_D, thus providing a quantifiable theoretical basis for the design of IGCTs against the dynamic avalanche. The comparison between simulation and experimental data shows consistent trends, verifying the validity of the model within the typical operating current range of the device. Among turn-off stresses, the turn-off current exerts the most significant influence on the triggering of dynamic avalanche: a higher turn-off current leads to a higher critical breakdown electric field, but the avalanche triggering voltage decreases rapidly. Among device structural parameters, N_D and α_pnp are the key parameters affecting dynamic avalanche triggering. A higher N_D increases the critical breakdown electric field for dynamic avalanche, yet the triggering voltage decreases. A larger α_pnp results in a decrease in both the critical breakdown electric field and the triggering voltage, making dynamic avalanche more prone to occur. The discrepancies between the model calculations and the device simulation and experimental test results imply that the current non-uniformity effects occurring during the IGCT turn-off will further advance the triggering instant of dynamic avalanche, and the temperature rise also affects the avalanche triggering instant.