The seepage of reservoir dam has always been a difficult problem in engineering construction, especially after the impoundment of dam. Limited by site or interference conditions, conventional nondestructive testing methods such as high-density electrical method and ground penetrating radar could not easily find the specific location and scale of leakage accurately. In this paper, the cross-hole ultrasonic transmission technology is introduced to detect the cracks in a dam. The first-break travel time of the obtained sound wave is picked up, and the wave velocity structure in the dam medium is obtained by using grid ray. The specific location and scale of dam cracks are determined with PSD (power spectrum density) discriminant method. The research results demonstrate that the cross-hole acoustic transmission imaging technology could detect the fracture development state of dam abutment by obtaining high resolution images. The detection results are highly consistent with the drilling data. In conclusion, it is proved to be an effective method for detecting reservoir dam leakage. The detection results offer detection basis for the treatment of leakage in large and medium-sized reservoir dam, and also provide reference for comprehensive assessment of reservoir dam risk consequences.