With a bandgap of 1.1-1.4 eV, Sn/Pb mixed halide perovskites are ideal materials for single-junction solar cells to reach the power conversion efficiencies (PCEs) limit of Shockley-Queisser (S-Q) theory. Their chemical composition gradient in the vertical direction of the perovskite films affect the transport and separation of carriers by changing the energy band structures. Therefore, it is very important to control the crystallization process of tin-lead mixed perovskite thin films. In this work, it was found that different vertical composition gradients were formed when tin-lead mixed perovskites were prepared with different amounts of the anti-solvent. Larger amounts of anti-solvent was contributed to higher lead content on the film surface. The vertical composition gradient of tin-lead mixed perovskite could be regulated by adjusting the solvent composition, among which increasing V(DMSO):V(DMF) in the solvent could form a vertical composition gradient with a lead-rich bottom and a tin-rich surface. When V(DMSO):V(DMF) in lead-based precursor solutions was optimized to 1 : 2, compared with the control group of 1 : 4, open circuit voltage of the device under standard light conditions increased from 0.725 to 0.769 V, short circuit current density from 30.95 to 31.65 mA·cm^-2, and PCE from 16.22% to nearly 18%. Numerical simulations using SCAPS further proved the necessity of forming a vertical composition gradient. When the bottom of the perovskite film is rich in lead and the top is rich in tin, the recombination of carriers in the hole transport layer interface region is reduced, which can improve the device’s performance.