Abstract: The Control rod drive mechanism（CRDM） is a key device in the nuclear reactor, and the pressure shell of CRDM is installed on the canopy of the reactor pressure vessel by the Ω seal weld. The primary water stress corrosion is the major factor for the cracking of Ω seal weld, resulting in the leakage of coolant. The structure integrity of the Ω seal can be insured by the overlay weld technology. Based on the fracture mechanics analysis method, this paper investigates the crack growth of the overlay weld structure on the Ω seal of control rod drive mechanism, induced by the fatigue and the stress corrosion. The calculation method and results can be a reference for the overlay weld structure design and assessment of the Ω seal in CRDM.