In situ crystallization of perovskites provides a promising approach to achieve high-quality semiconductor films ideal for optoelectronic devices. However, understanding of the complicated in situ crystallization processes of perovskites is still limited. Here, we report that controlling the solvent-vapor atmosphere in the spin-coating step leads to two distinct in situ crystallization modes of the perovskite films. A pure nitrogen atmosphere causes fast evaporation of the solvent of dimethyl sulfoxide (DMSO) and, in consequence, substrate-dependent crystallization processes. In contrast, an atmosphere with a controlled partial pressure of 15 Pa of DMSO slows down the evaporation rates, resulting in substrate-independent crystallization processes. The two in situ crystallization modes generate films of perovskite nanoplatelets exhibiting different thicknesses, optical properties, and hence, varied electroluminescent efficiencies. Our work suggests that the management of the solvent-vapor atmosphere during in situ crystallization of perovskite films provides a new dimension for controlling their structures and properties. Considering that solvent-vapor atmosphere is a basic parameter for all solution-based processes, our work is of general interest to the perovskite field.