In response to the identification of weak components within the life cycle of a mixed-connection system, this paper first extends the calculation method and properties of the Integrated Importance Measure (IIM) from a single time point to the entire life cycle. The extended IIM can identify components that have the greatest impact on the system during different periods or throughout the entire life cycle. Based on this, the paper provides the calculation formulas for the IIM of polymorphic series–parallel and parallel-series connection systems for component failure processes. Finally, the practicality of the extended importance measure formula is verified through numerical simulation.