During the operation of the ethylene cracking unit of a refining and chemical plant, the methane content at the top of the ethylene distillation column continuously increased. The investigation and analysis showed that it was caused by the leakage on the methane-hydrogen side of the condenser in demethanizer. After replacing the steel-aluminum transition joint with a new one, the methane content gradually decreased, meeting the production requirements. Through the operational analysis and the macro inspection, metallographic analysis, hardness testing and scanning electron microscope observations of the failed samples, it was found that the main reasons for the cracks include the improper control of the cooling and heating rates of cold box system during daily start-up and shutdown and the manufacturing defects in the steel-aluminum transition joint. By formulating strict operating procedures for cooling and heating rates, strictly refining and optimizing process operations, and optimizing the operating procedures for relevant equipment, the goal of long-term stable operation of the unit can be achieved.