Tin perovskites are emerging as promising alternatives to their lead-based counterparts for high-performance and flexible perovskite solar cells. However, their rapid crystallization often leads to inadequate film quality and poor device performance. In this study, the role of GeI2 as an additive is investigated for controlling the nucleation and crystallization processes of formamidinium tin triiodide (FASnI3). The findings reveal the preferential formation of a Ge-rich layer at the bottom of the perovskite film upon the introduction of GeI2. It is proposed that the initial formation of the Ge complex acts as a crystallization regulator, promoting oriented growth of subsequent FASnI3 crystals and enhancing overall crystallinity. Through the incorporation of an optimal amount of GeI2, flexible Sn perovskite solar cells with an efficiency of 10.8% were achieved. Furthermore, it was observed that the GeI2 additive ensures a remarkable shelf-life for the devices, with the rigid cells retaining 91% of their initial performance after more than 13 800 h of storage in an N2 gas environment. This study elucidates the mechanistic role of GeI2 in regulating the nucleation and crystallization process of tin perovskites, providing valuable insights into the significance of additive engineering for the development of high-performance flexible tin perovskite solar cells.