Calculation of cable force under seasonal ambient temperature is the key and difficult problem in the construction of long-span rib arch bridge. It affects the final cable forces and deformations of arch rib after arch closure. Unstressed state control method is introduced in the construction of Daning River Bridge; unstressed qualities of ribs and unstressed length of cables can be obtained by optimization analysis of the maximum cantilever state in construction. According to unstressed state control method, the cable forces of each segment were calculated using the forward-iteration method. These results offer the basis for construction control, and guarantee the arch rib line and cable forces to meet the design requirements, and an arch closure with high precision.