Metallized film capacitor (MFC) selection is a key step to ensure the safe and reliable operation of high-capacity power electronic equipment. However, as a crucial factor leading to overstress failure, the changing of stress tolerance boundary caused by parameter drift and insulation deterioration has not been fully considered in capacitor design. Therefore, the establishment method of MFC lifecycle safe operation area (SOA) is proposed innovatively in this article, which characterize the shrinking of safe operating boundaries caused by electrothermal coupling and capacitor aging during long-term operation. First, the failure mechanisms and electrothermal coupling of MFC are discussed and the modeling process of initial SOA is proposed based on the datasheet and dc withstand voltage experiments. In addition, considering the increase of the hot-spot temperature and the decrease of breakdown strength due to equivalent series resistance parameter drift and dielectric insulation degradation in the whole lifecycle, the shrinking of MFC operation boundary is characterized and the modeling process of lifecycle SOA is proposed based on aging model and lifetime tests. Finally, taking an MMC system as example, the MFC selection based on practical operating condition and lifecycle SOA is performed to verify the validity of the method.