Abstract: Existing studies on pore structures during coal spontaneous combustion mainly focus on low-temperature micropores, pore–fracture structures in single coal particles, or computed tomography (CT) structures of compacted coal columns, making it difficult to characterize the dynamic evolution of interparticle pores in loose coal particle assemblies during combustion. Two anthracites from the Rujigou Coal Mine in Ningxia (NX) and the Duanshi Coal Mine in Qinshui, Shanxi (SX) were selected, with particle sizes of 5–7 mm and 7–10 mm. Based on a visualized experimental system for loose coal spontaneous combustion, the dynamic evolution of mesoscale interparticle pore structures was investigated. An R-H-B stepwise flame–pore identification method integrating RGB and HSV color-space features was proposed, which effectively reduced the interference caused by high-temperature flame radiation and particle surface reflection, enabling the separate identification of dark pores, flame channels, and reflective regions. The results show that the mesoscale porosity of loose coal particle assemblies exhibits significant non-monotonic evolution during combustion: it first decreases and then increases during the heating stage, and increases overall with fluctuations during the combustion stage. The maximum porosity values of the four test conditions are concentrated within 0.46–0.48. Combined with porosity fluctuations and the response characteristics of connectivity parameters, this range can be regarded as a statistical reference interval for significant reconstruction of mesoscale interparticle pores. A segmented characterization model describing mesoscale porosity as a function of temperature was established, revealing the continuous response of porosity to temperature during the heating stage and early combustion stage. Particle size mainly affects the baseline porosity at low temperatures, whereas coal type mainly influences the temperature sensitivity of porosity; the NX coal sample at 7–10 mm shows the highest temperature sensitivity coefficient. Multi-parameter correlation analysis shows that the correlations among structural parameters vary significantly with combustion stage and particle size. Temperature is the dominant factor driving the dynamic evolution of mesoscale interparticle pore structures, with the temperature–porosity correlation coefficient reaching 0.73–0.82 during the intense exothermic combustion stage. Particle size determines the efficiency with which pore expansion transforms into dominant connected channels, and the correlation coefficient between porosity and the maximum connected-domain proportion reaches 0.50–0.69 in the 7–10 mm particle system, markedly higher than that in the 5–7 mm particle system. The results indicate that the mesoscale interparticle pore structure of loose coal during combustion is not stable, but undergoes dynamic reconstruction under the combined effects of temperature driving, particle collapse, and channel connection, and that static porosity alone cannot reflect its continuous evolution. These results provide experimental evidence for the continuous characterization of mesoscale interparticle pores during loose coal combustion and for analyzing the structural basis of local high-temperature zone formation.