Taking the dam section of the axial-flow unit hydropower house as the research object, the stress variation of draft tube and volute structure with different horizontal water sealing under operation and maintenance condition were analyzed. The results show that the tensile stress of the concrete and the steel lining in the draft tube section gradually decreased with the lifting of the water sealing, when the horizontal water sealing in transverse joint was placed in the range of the draft tube elbow section. When the horizontal water sealing in transverse joint was arranged within the volute elevation range, it was more effective than the closed solution to relieve the stress near the volute when connected with the upstream vertical water sealing. In the actual engineering project design, it is suggested that horizontal water sealing of transverse joint of the axial-flow unit hydropower house should be arranged as follows: the horizontal water sealing upstream section is arranged near the lower surface of the roof of the volute imported section and connected with the upstream vertical water sealing, and the horizontal water sealing downstream section is arranged near the top of the outlet of the draft tube divergence section.