Preliminary theoretical analyses indicate that lattice relaxation may be used to release lattice strain in the FAPbI ₃ perovskite to warrant both high x-ray detection performance and improved stability. Herein, we demonstrate stable black α-phase FAPbI ₃ single crystals (SCs) realized by lattice engineering via annealing in the ambient atmosphere. The engineered α-FAPbI ₃ SC detector shows almost all the best figures of merit including a high sensitivity of 4.15 × 10 ⁵ μC Gy _air ⁻¹ cm ⁻² , a low detection limit of 1.1 nGy _air s ⁻¹ , a high resolution of 15.9 lp mm ⁻¹ , and a short response time of 214 μs. We further demonstrate high-definition x-ray imaging at a dose rate below 10 nGy _air s ⁻¹ on the FAPbI ₃ SC, indicating a minimal dose-area product of 0.048 mGy _air cm ² to the patient for one-time posteroanterior chest diagnosis, which is more than 3000 times lower than the international reference level of 150 mGy _air cm ² . In addition, the robust long-term stability enables the FAPbI ₃ SC x-ray detector to work steadily for more than 40 years.