Organolead trihalide perovskites are emerging excellent active materials for thin film solar cells. Here, the formation and evolution of methylammonium lead iodide perovskite thin films grown by the low-temperature thermal annealing induced interdiffusion process is investigated. It is found that thermal annealing not only drives the formation of perovskite but also affects the morphology, optoelectronic properties and correlated device performance. The thermal annealing at 105 °C quickly drives the formation of phase-pure perovskite in a short time of 15 min, and followed thermal annealing up to two hours continuously increases perovskite crystallinity and grain size without interrupting the film continuity or coverage. Monotonically increased Hall mobility up to 36.0 cm2 V-1 s-1 correlates with the increased crystallinity and grain size upon annealing. Device efficiencies show an overall upward variation trend with increased short circuit current density and fill factor under longer annealing time up to two hours, and the highest device efficiency of 13.4% is achieved. It is found work function of the films decreasing with increased annealing time is linearly correlated with reduction of the open circuit voltage output, which points out a path for the further increase of the device efficiency.