Thickness of the soil layer on a hillslope is an important factor governing hydrologic processes on the hillslope. A field survey was carried out using a ProEx ground penetrating radar (GPR), product of MALA in Sweden, to detect thickness of the soil layer on a hillslope in a karst region. The detection used two antenna frequencies, 100 and 500 MHZ, separately, Then exploratory trenchess were excavated on the slope to determine thickness of the soil layer physically and at same time to explore for factors affecting accuracy of the radar detection, like weathering degree of the bedrock at the interface between the soil layer and the bedrock. On such as basis, a linear regression model and a GEP model was established with different independent variables to verify applicability of the ground penetrating radar to soil thickness detection on karst hillslopes. Results show that as in the karst region, the soil layers on hillslopes are generally thin and contain a lot of debris, the use of a higher frequency antenna may improve detection precision; the use of the combination of the results of the radar detections with two different frequencies and weathering degree of the bedrock as input variable is higher in simulation accuracy than the use of the results of the radar detection with a frequency as input variable. When input variables are the same, the GEP model is very limitedly higher than the linear regression model in prediction accuracy because of limitations in actual detection. So, the latter is recommanded. The linear regression model combining results of the detection using 100 and 500 MHZ antennas and weathering degree of the bedrock as independent variable may reach 0.660 and 15.0 cm in coefficient of determination and root mean square error, respectively.