Weathering degree of natural slope and its mechanical parameters have significant spatio-temporal variability. At present, the method of determining the mechanical parameters according to the suggestions of the specification ignores this phenomenon. In order to improve the accuracy of finite element numerical simulation results, a method for obtaining rock mass mechanical parameters based on ground penetrating radar (GPR) detection was proposed. Based on a rock slope of Xianyang international airport in Shaanxi province, the degree of fracture and water-rich inside the slope were detected by GPR, and the slope was divided into several units with different levels. Then, the mechanical parameters such as elastic modulus, Poisson's ratio and cohesion force of different units were obtained according to the specification. Fatherly, the finite element numerical simulation software MIDAS-GTS was used to establish the grid model of the slope, and the recommended values provided by the specification were used to calculate the sliding displacement of the slope. Compared with the field displacement monitoring data, the results showed that the numerical simulation values were closer to the monitoring values, indicating that the mechanical parameters obtained by this method were more consistent with the actual value of the rock mass.