A method for manufacturing large-area organic solar cells utilizes a hot solvent vapor annealing manufacturing process while manufacturing the organic solar cells via a large-area proceeding method, such as spraying. Namely, a heated solvent vapor is utilized to modify an active layer after the active layer of the organic solar cells is formed, which ensures a flatness and an uniformity thereof and increases a crystallinity of the active layer and an element charge transport rate so that a power conversion efficiency of the large area organic solar cells is increased, a proceeding time is quite short, and the performance thereof is quite obvious. Therefore, the method not only reduces the cost by a large area production but obtains organic solar cells with higher conversion efficiency.