Foreign Objects and Debris(FOD) may cause accidents or disasters，and seriously affect the safety of aircraft. According to the radar working in the fixed environment of the airport runway，the geometric model is established by using the coordinate system transform method. The approximation of model parameters is adopted in order to simplify the calculation. The statistical model is employed to produce clutter，and corresponding target signal model is established. The clutters mainly come from meadow and runways，therefore the backscatter coefficient of the composite scene is calculated with the combination of geometric models. The clutters of random sequences and echoes are simulated and the results are given.