Karst landforms are widely distributed in China, and are most common in Yunnan, Guizhou and Guangxi. If the development of karst caves at the bottom of the piles cannot be accurately ascertained before the construction of bridge pile foundations, accidents such as hole collapse, slurry leakage, and drill sticking will easily occur. In this paper, the principle and method of sonar detection for detecting karst caves at the bottom of bridge piles was introduced, and the sonar detection data and the cave situation at the bottom of the pile during the construction process in combination with the case of Yunnan Zhenguo Highway Project was analyzed, which verifies the practicability and reliability of sonar detection method reliability.