The technology of detection of internal cavities in concrete with laser-acoustic method is one kind of remote sensing detection technology for internal cavity of concrete, which has the characteristics of fast detection and non-contact, meeting the needs of identifying the internal cavity of concrete in some cases where it was not convenient to conduct contact detection. In this paper, focusing on the part of acoustic excitation, a laser acoustic concrete internal cavity detection system was built using accelerometers instead of laser vibrometers, and concrete specimens with internal prefabricated cavities were inspected. It was found that the presence of internal concrete voids would change the flexural rigidity of the structure above the voids, and when there was external excitation, the structure above the voids would show the phenomenon of bending vibration, the frequency of which was close to the first-order intrinsic frequency of the vibrating structure. The bending vibration in the defect area can be easily excited by using a high-power density pulsed laser, and when the cavity defect was shallow, the presence or absence of cavity inside can be judged by the characteristic response of the bending vibration in the frequency domain. As the depth of the cavity increased, the flexural rigidity of the structure above the cavity gradually increased, the amplitude of the bending vibration decreased, and the identification of the internal cavity will be difficult to be achieved by the characteristic frequency alone. In this case, the acceleration energy spectrum of the surface vibration signal can be used to characterize the vibration energy and determine whether there were cavities inside the concrete by the level of vibration energy. Based on the above theory, the cavity with a depth of 150 mm inside the concrete was successfully detected, which verified the feasibility of using laser acoustic technology to detect the cavity inside the concrete.