The posture of the bridge girder plays a very important role for the safety and efficiency of the bridge construction. This paper mainly proposed a mathematical method for determining the position and orientation of bridge girder and distributed Incremental Launching devices. Firstly, the background of incremental launching construction is introduced. In addition, the mechanical and hydraulic design of the launching device and its iterated construction movements are described. By analyzing the characteristics of incremental launching method and the distributed launching devices, the posture of single device and the bridge girder are determined mathematically. Further, the posture determination method is integrated into the bridge launching construction surveillance system. The application of this method is practically verified in the launching construction of Fengxi suspension bridge, finally.