During the start-up of an ethylene plant, the failure of reboiler in the demethanizer resulted in ultra-low temperature in the bottom of the tower, exceeding the design temperature of pipelines, and a series of secondary fluctuations during reboiling recovery such as the excessively high overhead temperature of the tower and the component disorder. The causes of these problems were analyzed, some improvement plans were proposed and technical modification was carried out to ensure the safe and stable operation of the plant. In view of the insufficient driving force of overhead condenser in the demethanizer, an optimization and improvement plan was proposed. After implementation, the overhead temperature reached the standard, the heat exchange effect was obviously improved, the ethylene loss at the top of the tower was significantly decreased and the ethylene recovery was improved.