Abstract: During the construction of large liquefied natural gas storage tanks, spot-like and floccus unknown images were found on the radiographic-inspection film of the 06Ni9DR steel weld by the ENiCrMo-6 nickel-based alloy electrode. The cause of unknown images was analyzed, and the effect of unknown images on the mechanical properties of weld was studied by microstructure observation, energy spectra analysis, hardness test, cryogenic impact tests, and thermodynamic calculations. The results show that the spot-like and floccus images were mainly caused by large-sized tungsten-rich inclusions and niobium-molybdenum-rich precipitate clusters. The average hardness of areas of inclusions and precipitate clusters were 456, 288 HV, respectively, which were both higher than that of the substrate; the tungsten-rich particles and precipitate clusters had a negative effect on the stability of tensile strength and cryogenic impact toughness of the weld joints. The quantity of unknown images decreased significantly after replacing the electrode. The unknown images were related to uneven mixing of coating components or uneven particle size of the flux during the production of nickel-based alloy electrodes, resulting in the abnormal enrichment of alloy elements in the weld metal.