To address the risk of short-term concentrated emission of pressure-relief gas and local methane overrun in the working face induced by key stratum breakage during the initial mining stage of an ultra-long working face in an outburst mine, this study proposes a technology for regulating the methane flow field during the initial mining stage through active key stratum breakage. Taking an outburst mine in Shanxi Province as the engineering background, similar simulation tests, PFC2D–COMSOL coupled numerical simulations, and field monitoring were comprehensively employed to investigate the breakage timing of key strata, the evolution of overburden pore–fracture structures, and the migration response of pressure-relief gas. The regulating mechanism of active key stratum breakage on the formation of pressure-relief gas migration channels and methane emission intensity was revealed. The results of similar simulation and numerical simulation show that, after active breakage was implemented for Sub-key Stratum 1, its initial breakage interval decreased from 35 m to 20 m, and the initial breakage timing was significantly advanced. Meanwhile, the pore-permeability distribution and fracture connectivity structure of the overburden were reconstructed during the initial mining stage, promoting the early formation of a dominant gas migration channel from the goaf and adjacent coal seams to the low-level roof roadway. This effectively inhibited the backflow of pressure-relief gas toward the working face and return airway, resulting in a substantial reduction in the peak methane concentration and overrun range at the upper corner of the return airway. The theoretical analysis indicates that active key stratum breakage reduces the gas accumulation scale and concentrated release intensity under large-area hanging roof conditions by shortening the initial breakage interval of Sub-key Stratum 1, thereby reducing the peak methane emission intensity during the initial mining stage by approximately 43%. Field engineering application shows that, after active breakage was implemented for Sub-key Stratum 1, the low-level roof roadway rapidly exerted its methane drainage effect during the initial mining stage, effectively preventing methane overrun accidents.