The driven polymer translocation through a nanopore with unbiased initial configuration has been studied by using Langevin dynamics (LD) simulations. It is found that the scaling relationship between translocation time and the polymer chain length is strongly affected by the friction coefficient in LD and the driving force. However, there is no scaling relationship between the translocation time and the friction coefficient. The translocation time is almost inversely proportional to the driving force, which is in agreement with those obtained in biased translocation. The scaling relationship between gyration radius (R-g) of subchain at the trans side with the subchain length (L) is R-g similar to L-0.33 that is in good agreement with the limiting value for molten globule state, while the curve of R-g of subchain at the cis side has two distinct stages. During translocation, the subchain at the cis side is being stretched gradually, and the structure of the subchain transforms from sphere-like to rod-like. When the effect of stretching reaches the tail end, the subchain is at the most stretched state. Finally the subchain will rapidly restore to coil structure. According to the results of force analysis, the retarding force at the trans side is more crucial during the practical translocation.