Cao river aqueduct is the mark building at main channel of the middle line of the South-to-North Water Diversion Project, of which the 30m span is the largest aqueduct in Asia for its span, and its structure is relatively complicated, which is the large-scale aqueduct that has been designed and constructed for the first time in China with great technical difficulties in structure form. For it will directly affect the safe operation of aqueduct that whether the actual stressing state, deformation conditions, prestress effect and etc. meet the design requirements, the impounding test of trial span was carried out before formal operation. As it can be seen from the test results analysis, aqueduct structure is under a good elastic working condition with about 1.00mm maximum deflection at mid-span and meets the design and specification requirements, which means the structure of aqueduct is reasonably designed, and structure operation is safe and reliable. In the meantime, the stress characteristics and deformation law of aqueduct structure has been mastered more overall and deeply according to the research on the impounding test, which provides important reference for later operation and management.