Abstract: The article analyzes the process of filling water and heat preserving and pressure loading & unloading to the spiral case. Loading a pressure to the spiral case, the tension stress of steel plate increases; while unloading, total changing regulation of all instruments shows that the tensile stress decreased,and the joint between the spiral case and the concrete enlargeed,and the compressive stress of the perimetric concrete and steel bars increased.After unloading,the maximum joint took place in the cool seasons, and the minimum joint in the summer. Through analysis, we consider that the spiral case of left powerhouse units works normally, and that the stress and deformation are within the range of allowable design.