Individuals have different rumination patterns after experiencing traumatic events in different cultural backgrounds and situations. This study was aimed to explore the experience of Chinese young and middle-aged patients with acute coronary syndrome (ACS) to understand the content and nature of their rumination. Fourteen participants were selected using the purposive sampling method in XXX hospital from May 2023 to December 2023. Data were collected using semi-structured interviews and analyzed using Colaizzi’ s 7-step phenomenological method. The content of rumination in young and middle-aged patients with ACS can be summarized into 2 categories: non-adaptive rumination and adaptive rumination. Non-adaptive rumination included 3 themes: worry or anxiety, scare, and depression. Adaptive rumination included 4 themes: tracing of disease processes, enhancement of disease cognition, improvement of health awareness, and adjustment of lifestyle cognition. In conclusion, although Chinese young and middle-aged patients with ACS experience negative emotions after a traumatic cardiac event, they gradually make positive changes, and optimism and information support play important roles in this transition, which provide a fundamental understanding of rumination experiences in Chinese young and middle-aged patients with ACS and provide new data for healthcare providers when designing intervention programs to enhance post-traumatic growth in these patients.