This paper establishes a mechanical equilibrium equation for structure in any construction process of bridge formed by stages, and thus concludes that the core of process control for stage-constructed bridge is the quantity of unstressed state of structural elements. The control and adjustment of quantity of unstressed state of the structural elements provides a solution to the problem of rection calculation for bridge constructed by stages. Based on the principle that internal force and displacement have nothing to do with the construction process on the premise of a constant quantity of unstressed state, parallel operation of multiple working procedures can be put into practice, and the automatic filtration of influences from temporary load and temperature during construction can be therefore well realized.