The invention discloses a detection system and a detection method for a concrete dam. The detection system comprises a data acquisition processing platform, a vibration source module and a hydrophonemodule, wherein the vibration source module is controlled to striking a downstream surface of a dam, a signal is received by an optical-fiber hydrophone in the upstream water, the one-transmission multi-receiving is realized in the test process, the data acquisition processing platform records a plurality of groups of transmitting vibration source signals and hydrophone receiving signals on each measuring point of the concrete dam, records a space coordinate space of a vibration source point and a hydrophone point, and acquires the test data to calculate the elastic wave trend between all space measuring points of the concrete dam, so that a low wave velocity area or a low wave velocity band can be determined, a CT image of the detection fault can be formed, the multi-fault detection can be performed for cracks, holes, aged concrete or other defect features inside the dam body according to the requirement, and finally the three-dimensional dam CT image can be formed, thereby providingscientific evidence for the next repairing and reinforcement.