Perovskite solar cells (PSCs) have become a research hotspot in the photovoltaic field due to their excellent photoelectric conversion efficiency (PCE), simple preparation process, and low-cost characteristics. Among them, flexible perovskite solar cells (FPSCs) possess unique advantages such as lightweight and wearability. Compared with rigid devices (RPSCs), the efficiency improvement of FPSCs faces multiple challenges: the limitations of the light transmittance of flexible substrates, insufficient temperature and weather resistance, as well as interface defects and mechanical stress damage caused by low-temperature preparation processes, which severely restrict the device performance and stability. This paper systematically reviews the latest research progress in interface engineering and defect passivation strategies for PEN/ITO flexible substrates and n-i-p device structures. It focuses on key technical routes such as surface reconstruction of functional layers, chemical regulation of grain boundaries, and modification of heterojunction interfaces. Through strategies such as the introduction of novel passivation molecules, self-assembled monolayers, and gradient energy level design, the charge transport and recombination characteristics have been effectively improved. In addition, regarding the large-scale preparation technology of flexible perovskite modules, the optimization schemes for large-area film formation processes are summarized, and the transition from small-scale devices to large-area photovoltaic modules for various applications is discussed. The methods for stress control and defect suppression during the R2R manufacturing process are analyzed. This study has important reference value for promoting the industrial application of flexible perovskite photovoltaic technology.