To study different influences of original water tables on the roadbed moisture field infiltrated by rainfall, five scale-down laboratory roadbed physical models with different original water tables were tested base on saturated-unsaturated soil seepage theory. The roadbed moisture field change rules were examined and its mechanism was explained through analyzing infiltration test results of two different rainfall intensities. The results show that the water tables rise significantly with the increase of rainfall strength, and the influences of original levels on moisture fields get more significant along with the increase of roadbed depths. With decrease of original levels the amplifications of roadbed moisture range increase quadratically to a steady value and the extended distances of moisture disturbed zone decrease quadratically to a steady value. A balanced original water table should exist according to a given field situation. The influences of groundwater on moisture fields are very slight while the original levels are deeper than the balanced one, otherwise, with increase of original levels the amplifications of roadbed moisture increase logarithmically and the influence is very significantly when the water table is higher than the balanced one.