Floor structure heaving and lining cracks occurred at an operating cable tunnel of Huachang Substation in Dalian, China. These disasters of the cable tunnel are detected by means of site measuring and core drilling; and then the causes of the disasters are analyzed. It is concluded that the groundwater, the insufficient strength of the primary supporting structure and the bad construction quality comprehensively give rise to the tunnel diseases. Countermeasures, such as temporary translocation of the cables, cutting of floor after borehole drilling and the reinforced concrete lining, are taken. The practice shows that the tunnel structure is stable and the cable runs just fine after using the abovementioned measures.