Abstract: Because of the complexity of geological conditions of tunnel surrounding rock and the limitation of site investigation before tunnel construction,the geological prospecting results usually do not agree with the real conditions. Consequently,advanced detection methods must be adopted in tunnel engineering in order to insure the construction safety. The paper presents the use of the ground penetration radar SIR-3000 system in Mangtouling tunnel along the Chenzhou-Ningyuan express highway. The data was processed with the spatial filter and Hilbert transform. Karst cave and the cranny were forecasted in the front of tunnel face by GPR results. The excavation result testified the validity of the forecasted results. A better forecast effect was obtained. The result illuminates the serviceability of spatial filtering and Hilbert transform for GPR data processing.