Due to the impact of climatic and natural factors, the normal-speed railway subgrade is vulnerable to mud pumping, water accumulation, subsidence and other defects, hereafter called diseases, that frequently occur, after a period of service. It is imperative to quickly detect subgrade diseases. On the basis of studying the law of geological radar electromagnetic signal propagation along the railway subgrade, according to the results of site excavation and verification, this paper proposes a method for railway subgrade disease inspection based on ground penetrating radar. In addition, radar image features of typical subgrade diseases are clarified herein. Practical application results show that this inspection method is effective to accurately identify subgrade diseases. In particular, it is of great significance to the rapid inspection of hidden subgrade diseases. In addition, an analysis of inspection results and distribution of subgrade diseases along the line is conducive to determining the key section for treatment of subgrade diseases.