Black‐phase formamidinium lead iodide (FAPbI3) with narrow bandgap and high thermal stability has emerged as the most promising candidate for highly efficient and stable perovskite photovoltaics. In order to overcome the intrinsic difficulty of black‐phase crystallization and to eliminate the lead iodide (PbI2) residue, most sequential deposition methods of FAPbI3‐based perovskite will introduce external ions like methylammonium (MA+), cesium (Cs+), and bromide (Br–) ions to the perovskite structure. Here a zwitterion‐functionalized tin(IV) oxide (SnO2) is introduced as the electron‐transport layer (ETL) to induce the crystallization of high‐quality black‐phase FAPbI3. The SnO2 ETL treated with the zwitterion of formamidine sulfinic acid (FSA) can help rearrange the stack direction, orientation, and distribution of residual PbI2 in the perovskite layer, which reduces the side effect of the residual PbI2. Besides, the FSA functionalization also modifies SnO2 ETL to suppress deep‐level defects at the perovskite/SnO2 interface. As a result, the FSA–FAPbI3‐based perovskite solar cells (PSCs) exhibit an excellent power conversion efficiency of up to 24.1% with 1000 h long‐term operational stability. These findings provide a new interface engineering strategy on the sequential fabrication of black‐phase FAPbI3 PSCs with improved optoelectronic performance. A zwitterion formamidine sulfinic acid (FSA) is deposited at the tin(IV) oxide (SnO2)/perovskite interface to induce the crystallization of high‐quality black‐phase formamidinium lead iodide (FAPbI3), and also help reduce the side effect of the residual lead iodide (PbI2), yielding an impressive power conversion efficiency (PCE) of 24.1% and 1000 h long‐term operational stability.