Developing in situ-grown perovskite single-crystal thin films (PeSCTFs) on transport layers is essential to achieve high-performance optoelectronic devices. However, it remains a challenge to grow PeSCTFs in situ with a high area-to-thickness ratio and low trap density. Here, we propose a new strategy of gradient heating nucleation and room-temperature growth. Gradient heating nucleation helps to overcome the high nucleation energy barrier and reduce the number of crystal nuclei, and room-temperature growth helps reduce the crystal growth rate and aims at an orderly deposition and diffusion of atoms for in situ growth of PeSCTFs with high quality. Consequently, the fabricated FAPbBr₃ PeSCTF achieves a record area-to-thickness ratio of 1.84 × 10⁵ mm and a low trap density of 3.76 × 10⁸ cm⁻³. Moreover, the proposed strategy is also shown to be a universal method for in situ growth of other kinds of PeSCTFs on multiple transport layers.