Single crystalline structures with less grain boundaries and longer carrier diffusion length are essential for high performance perovskite solar cells. In this paper, we report a molten salt-assisted method to gain a perovskite film with submillimeter-sized single crystal domains by using excess ammonium salt for the first time. The resulted crystal size can reach 0.5 mm, and an aspect ratio over 1000. A photovoltaic device based on this film exhibits a champion power conversion efficiency (PCE) of 10.12%. This synthetic strategy enables the formation of submillimeter-sized perovskite films first, which holds promise for the understanding of crystallization in organic-inorganic perovskite and paves the way to enhance the efficiency of perovskite solar cells.