The invention discloses an outer-side span stay cable-free stay bridge with horizontal stress performance improved. A girder is supported by bridge towers, auxiliary piers and transition piers. The auxiliary piers are arranged on two sides of the bridge towers, and the transition piers are arranged on two sides of the auxiliary piers. The portion, between the auxiliary piers and the transition piers, of the girder is connected with columns of the bridge towers without stay cables. The girder is supported by the transition piers through first vertical support seats with horizontal unlocking functions. The auxiliary piers support the girder through second vertical support seats and are connected with the same through horizontal damping devices. Damping direction of the horizontal damping devices is the horizontal direction of the girder. Lower beams of the bridge towers support the girder through third vertical support seats. The girder is connected with the columns of the bridge towers through horizontal support seats which are horizontally distributed along the girder. According to the arrangement, stress applied to the auxiliary piers, the transition piers and the base is greatly decreased under horizontal earthquake action, the stress to the auxiliary piers and the transition piers is harmonized, and horizontal displacement occurring in case that the auxiliary piers are under the earthquake action, of the girder is under control.