Abstract: To study the dispersion of low-temperature fracture toughness and the discontinuous “Pop-in” phenomenon of multi-layer and multi-pass welds using Chinese welding materials for marine structural steel, the causes of the Pop-in phenomenon in the weld metals using Chinese welding materials were analyzed by comparing the fracture morphology, microstructure, EDSD crystallographic characteristics, and crack propagation paths of crack tip opening displacement (CTOD) specimens with two different thicknesses. The test results indicate that at − 10 °C, no Pop-in effect appears in the fracture toughness tests of the 60 mm-thick weld metals, and all CTOD values are above 500 μm; whereas the Pop-in effect occurs in all 100 mm-thick specimens, resulting in low fracture toughness with CTOD values of less than 100 μm. The microstructure of the 60 mm-thick specimens is composed of acicular ferrite and proeutectoid ferrite, while lath bainite appears in the microstructure at the mid-thickness position of the 100 mm-thick specimens. Moreover, there are few high-angle grain boundaries and abundant low-angle grain boundaries in the microstructure, which reduces the ability to hinder crack propagation and easily causes local microstructural embrittlement. The main reasons for the Pop-in effect in the 100 mm-thick weld metal CTOD specimens are the coupling of the thickness effect, local microstructural embrittlement, and inclusions.