In order to acquire high resolution GPR exploration data, we should not only adopt suitable methods to get high resolution data in the open field but also process primitive data with essential methods. The precision of the traditional method for complex trace analysis cannot meet the requirement of the high resolution GPR exploration, especially that of the thin interbeds research. Therefore an improved method for complex trace analysis is proposed in this paper to suit the high resolution GPR data. The traditional instantaneous phase plot is compared with the improved instantaneous phase plot based on exploration data of Shanghai-Nanjing Railway subgrade, and the results demonstrate that the improved method of complex trace analysis is more precise and effective than the traditional one.