It has been indicated, during FWD test, that variations of loading time and time integration of load have influences on the estimated elastic modulus and K-value of subgrade even if the peak load is same. The above indication could not be found from the impact numerical FE analysis of elastic model for the portable FWD. In this study, analytical study has been carried out to investigate the influences of various parameters, such as the rigidity of subgrade, the mass and drop height of falling weight, on the loading time and time integration of load from the numerical analysis of viscoelastic subgrade model.