Abstract: 30CrMnSi high-strength steel is widely used in the manufacturing of heavily loaded, high-strength components and structures. However, this material is characterized by a high carbon equivalent (approximately 0.7), a strong hardening tendency, and susceptibility to defects such as cold cracks and porosity. This study addresses the technical challenges of electron beam welding for circumferential seams with a locked-back joint in 38 mm thick 30CrMnSi steel plates. Using an SEBW(G)80-20B vacuum electron beam welding system, the influence of key parameters—accelerating voltage, beam current, welding speed, focusing current, and scanning waveform—on weld formation and quality was systematically investigated. The welding process was supported by auxiliary measures including preheating, tack and sealing welds, post-weld heat preservation, and heat treatment (holding at 620°C for 3 h). Comprehensive evaluation of the welds was conducted through visual inspection, ultrasonic testing, phased array testing, X-ray inspection, and metallographic analysis. The results demonstrate that optimized process parameters (accelerating voltage 60 kV, beam current 220 mA, welding speed 70°/min, focusing current 180 mA, with circular scanning) can produce high-quality welds with good formation, absence of "pearlite leakage" defects, and slight penetration at the locked-back joint. Batch verification showed a weld qualification rate of 90% and a tensile strength exceeding 1500 MPa, meeting the Class I weld requirements of the GJB-1718A standard. The established process solution has been successfully applied to actual product manufacturing, providing reliable technical guidance for welding similar high-strength steel structural components.