To monitor the change of the internal defect degree and locate the defect position of the concrete structure in service, this paper explored a detection method based on stress waves and carries out experimental research. Piezoceramics transducer (PZT) was installed on the concrete surface in the form of an array, and the concrete was divided into different areas. One sensor in the array emitted waves and the other received waves, and sweep frequency mode and five peak pulse modes were used to monitor different areas of concrete. The test results show that when the stress waves pass through the area to be monitored, due to the existence of internal defects in the concrete, the amplitude value of the stress waves attenuates and the wave energy decreases. In order to quantify the degree of internal defects in concrete structures, the energy index and damage index based on the received signal are constructed and their functional relationships with the degree of defects are established, respectively. According to the difference between the energy index and damage index in different areas, the areas where internal defects exist in concrete are determined. The test results are consistent with the actual situation of the test pieces, which verifies the feasibility of using piezoelectric sensors to monitor the internal defects of concrete in this paper.