Addressing the toxicity issue in lead-based perovskite compounds by seeking other nontoxic candidate elements represents a promising direction to fabricate lead-free perovskite solar cells. Recently, Cs<sub>2</sub>AgBiBr<sub>6</sub> double perovskite achieved by replacing two Pb<sup>2+</sup> with Ag<sup>+</sup> and Bi<sup>3+</sup> in the crystal lattice has drawn much attention owing to the convenient substitution of its chemical compositions. Herein, the dependence of the optoelectronic properties and corresponding photovoltaic performance of Cs<sub>2</sub>AgBiBr<sub>6</sub> thin films on the deposition methods of vacuum sublimation and solution processing is investigated. Compared to the vacuum sublimation based one, the solution-processed Cs<sub>2</sub>AgBiBr<sub>6</sub> shows inherently higher crystallinity, narrower electronic bandgap, longer photoexcitation lifetime, and higher mobility. The excellent optoelectronic properties are attributed to the accurate composition stoichiometry of Cs<sub>2</sub>AgBiBr<sub>6</sub> films based on solution processing. These merits enable the corresponding perovskite solar cells to deliver a champion power conversion efficiency (PCE) of 2.51%, which is the highest PCE in the Cs<sub>2</sub>AgBiBr<sub>6</sub>-based double perovskite solar cells to date. The finding in this work provides a clear clue that a precise composition stoichiometry could guarantee the formation of high quality multicomponent perovskite films.