Due to the long-term geological process and external factors, the bridge foundation as a key component of bridge always tends to get some diseases. Considering that the traditional common-offset method cannot detect bridge foundation defect very well, this paper proposes the use of a special common-source method to detect the bridge foundation defect. The forward modeling of the bridge foundation detection is realized by GprMax. The differences can be found easily by comparing the result of normal bridge foundation and that of defective bridge foundation. The result obtained by the authors is helpful to identifying the defective bridge foundation, which proves that GPR is an effective method for detecting the defect of bridge foundation.