Based on characteristic of relevance of wave form and energy of effective signal among neighbouring traces,the author fills up the deficiency in extraction of horizontal coherent signal with traditional KL transform and uses KL transform in wavelet domain to filter GPR data.Not only are axis of tilt and bending axis intensified,but also signal is effectively distinguished from interference signal whe signal-to-noise ratio is low.So the resolution of GPR plot is improved.After the GPR data from exploration of Shanghai-Nanjing railway subgrade is filtered with the method mentioned above,it is easier for the GPR plot based on it to reflect the subgrade problems.