Abstract: Numerical simulation methodologies were employed to investigate the characteristics of spiking defects during 5B70 aluminum alloy electron beam welding, along with their formation process and correlations with molten pool dynamics and keyhole evolution. A ray tracing-based heat source model for electron beam welding was proposed, which incorporated the attenuation of electron beam energy density within metal vapor, and then a comprehensive welding flow field model was established, accounting for dominant forces including surface tension, gravity, buoyancy, recoil pressure, and Marangoni convection. The causes of spiking defect formation were systematically analyzed. The results have shown that the dynamic evolution of spiking defects is influenced by the combined effects of liquid metal flow, temperature variation, and the advancement rate of the solid-liquid interface. The irregular inner wall morphology of spiking defects originates from non-uniform solidification behavior induced by differential cooling rates and liquid flow patterns. The limited volume and poor fluidity of liquid metal at the molten pool bottom contribute to the preferential occurrence of spiking defects at the weld root region. By keeping the keyhole open through appropriate techniques and reducing the temperature gradient at the bottom of the molten pool, the stability of the keyhole can be enhanced, and the fluidity at the bottom of the molten pool can be improved. Then the occurrence of spiking defects can be suppressed.