Fibrous filters are widely used for aerosol control; however, the coupled effects of elliptical fiber geometry and orientation on diffusion-dominated nanoparticle capture and efficiency–resistance optimization remain insufficiently understood. This study develops a numerical framework to investigate the effects of fiber aspect ratio (ε = 1–4), attack angle (θ = 0–90°), packing density (C = 0.01–0.10), and particle diameter (d_p = 0.01–1 μm) on the diffusional collection efficiency, pressure drop, and quality factor of elliptical fibers. The airflow field is solved under steady laminar conditions, and particle trajectories are simulated using a Langevin-based Brownian motion model. The results show that elliptical fibers with moderate aspect ratios (ε ≤ 2) exhibit filtration characteristics comparable to circular fibers, whereas highly elongated fibers (ε = 4) provide enhanced particle capture by modifying near-fiber flow structures and increasing particle–fiber interactions. However, the benefits of increased aspect ratio depend strongly on fiber orientation and must be balanced against the associated pressure-drop penalty. Quality factor analysis reveals that the streamwise-aligned elliptical fiber configuration (ε = 4 and θ = 0°) achieves the best overall performance within the investigated parameter range by maintaining high collection efficiency while minimizing aerodynamic resistance. These findings demonstrate that elliptical fiber optimization requires simultaneous consideration of geometry and orientation and provide mechanistic guidance for designing energy-efficient fibrous filtration materials.