For different types of the railway and the highway long-span prestressed concrete bridges,the construction method of building bridges section by section has been widely adopted, in which the deflection and the pre-arched calculation and installation are the key techinque for engineering control. The paper analyzes the current pre-arched calculation and installation method based on the relative deflections,and proposes a new absolute deflection method. The the- oretical analysis and pratical applications proof that the new method is proper and reliable, easy to be applied and popularized,and able to raise obviously control quality of building bridges.