Combined with the real example of monitoring pressure on the supporting system in the long span underground space of DK7+692 section at Jiao-Xin line of Chongqing light railway,it is set forth the methods about choosing the support system and its parameter,digging methods of underground space,measuring and testing methods for the stress of the support system;the testing results and variation pattern were analyzed for the stress of the initial supporting I-steel,axial stress of bolt,stress of shot-concrete,supporting pressure of temporary I-steel and stress of steel of the second lining,then,the reason for deformation of the support system induced by sharp increase of I-steel was determined.Based on this,considering the real problem,the construction plan and the support system is changed,the deformation of support system was under control and the danger was avoided,the informative construction and construction safety were realized.