Abstract: Objective The aim was to address the challenge of welding liquation cracks in domestically produced Fe-Ni-based cast superalloys used in advanced ultra-supercritical power generation equipment operating at 700 ℃. Methods This paper systematically investigates crack characteristics and formation mechanisms through techniques such as optical microscope, scanning electron microscope, energy dispersive spectroscopy, and ultra-high temperature laser confocal microscope in-situ observation. Results The results indicate that MC-type carbides and enrichments of C, Mo, Ti, and Al elements are present near the liquation cracks in the alloy. The alloy liquefies in the temperature range of 1 280～1320 ℃, with the carbide-matrix interface serving as the initiation site for liquefaction, and the carbides remain in the solid state throughout the process. Conclusion The essence of crack formation lies in the enrichment of solute elements at the carbide-matrix interface, which induces constitutional liquation to form a liquid film. This film then tears under thermal stress, leading to crack initiation. This study provides a theoretical foundation for optimizing the crack resistance of Fe-Ni-based cast superalloys and their engineering applications.