In this study, we provide a method of adjusting grain size in vapor deposition method. The grain size optimization of MAPbI₃ films is realized by simply modulating the reaction temperature between PbI₂ films and MAI vapor. We set the reaction temperature to be 140 ℃, 160 ℃, 180 ℃ and 200 ℃ separately and establish the relationship between reaction time and grain size during the complete conversion of PbI₂ film into MAPbI₃ film. We find that the average grain size of the film increases first with growth temperature increasing from 140 ℃ to 180 ℃ and then decrease at 200 ℃, giving an average grain size of 0.81 ${\text{μm}}$ and a largest grain size of about 2 ${\text{μm}}$ at 180 ℃. The defect density of perovskite film is deduced from the space charge limited current model, showing that it decreases from 5.90 × 10¹⁶ cm^–3 at 140 ℃ to 2.66 × 10¹⁶ cm^–3 at 180 ℃. Photovoltaic devices with structure FTO/TiO₂/C₆₀/MAPbI₃/spiro-OMeTAD/Au are fabricated to demonstrate the performance. It is found that the devices with an active area of 0.045 cm² show that with the increase of grain size, the average PCE increases from 14.00% to 17.42%, and the best device shows that its PCE is 17.80% with 4.04% hysteresis index. To show the possibility of scaling up, we fabricate a uniform perovskite thin film with an area of about 72 cm², and a device with an active area of 1 cm², which gives a PCE of 13.17% in reverse scan. In summary, our research provides a method of regulating the grain size for the vapor deposition, which can improve device performance by reducing the trap density in perovskite film for suppressing the carrier recombination in grain boundary. Meanwhile, we prepare high performance devices and large area thin films, showing their potential in large area device fabrication and applications.