For long-distance deep mining in metal mines, the adoption of tandem attached jet ventilation leads to significant air quality attenuation and easy cyclic accumulation of pollutants. This study employs CFD numerical simulation methods to construct a multidimensional pollutant diffusion model of airflow and wet-thermal-dust under tandem ventilation conditions. After in-depth analysis of airflow structure and pollutant diffusion patterns, a dual-stage variable diameter ventilation optimization design is proposed, featuring large-diameter air duct conditioning in the first section and localized pressurization in the secondary section. Key parameters (duct diameter ratio λ and tandem spacing H) are further optimized. Experimental validation in a deep metal mine long-distance roadway confirmed the following findings: the tandem attached jet airflow can be divided into four zones: the jet development zone, deflected airflow zone, recirculation zone, and stable backflow zone. The strong shear vortex structure formed in the recirculation zone causes the aggregation and swirling of comprehensive thermal-dust pollutants in the roadway, which is the key cause of secondary pollution. Under the dual-stage variable diameter ventilation mode, the pollutant removal capacity in four experimental groups (λ=0.75, 0.80, 0.85, 1.00) initially strengthens and then weakens as tandem spacing H (0.5 m≤H≤1.0 m) decreases. Under the same H condition, a similar trend of initial increase followed by decrease is observed as λ decreases. Within the current research range, an optimal operating condition (λ=0.80, H=1.0 m) exists. Compared to pre-optimization, post-optimization results show a 45.8% shortening of the time to reduce breathable dust concentration to 1 mg/m3 in the working zone, along with a 35.4% reduction in the connecting zone. This effectively enhances pollutant emission capacity under tandem ventilation conditions.