In order to study the influence of the pre-stressing tension on the bridge force under complex climatic conditions in mountain areas and monitor in real time the concrete stress of long-span bridge in the process of pre-stressing tension, by taking the construction of Wanlongshan Bridge as an example, plastered fiber grating sensor with high-accuracy was adopted to monitor the whole process of pre-stressing tension in Block 12 of Pier 2. The research results show that when the prestressing tension is carried out by using the asymmetrical tension method, the phenomenon of alternating tension-compression strains occurs in the box girder section. When the tension occurs in the roof, the bottom board and web produced obvious tension stress, and the web of the other side without tension also produced obvious tension stress; the prestressed steel beam of the roof produced more obvious prestressing force than that of the bottom board. After the prestressing of 12 # block was completed, a compressive strain of -120με was produced at the top of box girder section, and tensile strain of 40με and 80με were produced respectively at web and bottom board, which directly reflected the effect of stress tension.