Abstract: In view of the dispersion and Pop-in phenomenon of low-temperature fracture toughness of multi-layer multi-pass welds of domestic welding material for marine structural steel, this paper analyzes the causes of the Pop-in phenomenon of domestic weld metal by comparing the fracture morphology, microstructure, EDSD crystallographic characteristics and crack propagation paths of CTOD specimens with different thicknesses. The test results show that , there is no Pop-in phenomenon and the CTOD value is above 0.5mm in the fracture toughness test of 60mm thick welds at -10 ℃, while the Pop-in phenomenon is shown in 100mm thick samples, resulting in low fracture toughness and CTOD value of less than 0.1mm. The structure of the 60mm thick specimen is acicular ferrite and polygonal ferrite, while the structure of the 100mm thick specimen is lath bainite in the middle thickness position, and there are few high angle grain boundaries and many low angle grain boundaries in the microstructure, which reduces the ability to hinder crack propagation and easily causes local microstructure embrittlement. The main reasons for the Pop-in phenomenon of 100mm thick weld metal CTOD specimens are the coupling effect of thickness effect, local microstructure embrittlement and inclusions.