In response to the frequent chain accidents of leakage–freeze expansion–fire in domestic ethylene plants with ultra-long service cycle (>30 years) during winter, this paper takes a rupture-and-fire incident on the drain line of an ethylene cracking unit as the example, and reconstructs the release sequence using multi-source data from DCS, GDS, weather stations and surveillance cameras. Using Phast software, a three-stage leakage model of water → liquid hydrocarbon → cracked gas is established and coupled with video frames. It is confirmed that, under conditions of -15 ℃ heat-tracing failure and CUI wall-thinning to 2.06 mm, the DN50 blind-end pipe suffers a 45° shear rupture when internal pressure exceeds the freeze-expansion limit. A four-dimensional closed-loop management strategy of design–inspection–warning–emergency for winter operation of aged units is proposed, providing quantitative bases for standard upgrades of similar installations in the industry.