Supersonic underexpanded jet impinging on an inclined flat plate is computationally simulated using RANS and ILES. In this paper, validations of these simulations for this flow field are conducted and the results show that simulations are qualitative agreeements with experiment. Furthermore, the factors leading to the localized pressure peaks on the plate surface and the unsteadiness of these pressure peaks are discussed in detail. In the present condition, there are 3 factors leading to the pressure peaks: strong shock waves in the upstream area, stagnation point of the main stream in the upstream area, interaction between the intermediate tail shock and the boundary layer. In addition, these pressure peaks have some unsteadiness respectively.