<p>Breakdown strength plays a crucial role in determining the efficiency and reliability of polymer dielectrics in electric power systems. However, elucidating their multiscale breakdown mechanisms, especially at the microscopic scale, remains a fundamental challenge. Here, we develop an atomic-scale electron avalanche breakdown model, coupling carrier transport, impact ionization, and chemical bond evolution, to investigate the dynamic behaviors of electrons in breakdown process of various polymer systems. Our findings demonstrate that highly ordered crystalline phases effectively suppress electron acceleration, whereas disordered molecular chain segments within amorphous regions are more prone to inducing along-chain electron avalanches. Furthermore, we identify that carrier kinetic parameters play a decisive role in governing the dielectric breakdown strength of polymers. Building upon this insight, we evaluate polymer blends focusing on the band gap (<InlineEquation ID="IEq1"><EquationSource Format="TEX">\({E}_{g}\)</EquationSource><EquationSource Format="MATHML"><math><msub><mrow><mi>E</mi></mrow><mrow><mi>g</mi></mrow></msub></math></EquationSource></InlineEquation>) and the mean free path (<InlineEquation ID="IEq2"><EquationSource Format="TEX">\(\lambda \)</EquationSource><EquationSource Format="MATHML"><math><mi>λ</mi></math></EquationSource></InlineEquation>), revealing that the impact of blending on breakdown strength is fundamentally a result of the competition between band matching and electron scattering. Unlike global band shifts in blends, small-molecule incorporation introduces localized deep traps that effectively capture free carriers, thereby enhancing breakdown strength. Using high-throughput calculations, we establish the relationship maps between band gap, electron mean free path, trap depth, and breakdown strength in polymer incorporated with small-molecule, revealing that breakdown is acutely sensitive to deep traps (activation energy &gt; <InlineEquation ID="IEq3"><EquationSource Format="TEX">\(2 \sim 3{\rm{eV}}\)</EquationSource><EquationSource Format="MATHML"><math><mrow><mn>2</mn><mo>~</mo><mn>3</mn><mi mathvariant="normal"> eV</mi></mrow></math></EquationSource></InlineEquation>). The atomic-scale understanding of electron avalanche breakdown process provides more refined guidance for molecular/defect engineering to transition the development of high-performance dielectrics from empirical trial-and-error to rational design.</p>