Through macro-observation，metallographic analysis，fracture surface examination，energy dispersive spectroscopy，and thermal simulation testing，an in-depth investigation was conducted on vacuum electron beam welds and diffusion welding cracks in stainless steel micro-flow controllers.Results indicate that the primary cause of welding cracks is the presence of phosphorus-containing low-melting-point eutectic phases generated by diffusion welding on the butt joint surfaces of vacuum electron beam welds.These low-melting-point eutectic phases not only increase phosphorus impurities within the weld pool，triggering crystallization-induced hot cracks，but also melt within the heat-affected zone of the vacuum electron beam weld，leading to diffusion weld cracking.Furthermore，employing a modified welding method with a large defocusing amount for the vacuum electron beam weld resulted in a wider weld with shallow penetration.The resulting columnar crystals grew nearly parallel to the weld joint plane，reducing the weld''s ability to resist tensile stresses during solidification and further increasing the risk of crystallization hot cracking initiation and propagation.Ultimately，during leak testing，crystallization-induced thermal cracks originating at the junction between the vacuum electron beam weld and the diffusion weld propagated outward，causing leakage.