In the foreland foldthrust zone in the southwestern margin of the Tarim basin develops two types of different tectonics: passive-roof duplex in the Fusha area and piggyback basin in the Wupoer area. In order to investigate their origins, a series of two-dimension numerical modeling was conducted using finite difference program FLAC. Through following the visco-elasto-plastic constitutive model of plane strain, two detachment layers and three competent layers were designed. In combination to basement subsidence, syntectonic erosion and sedimentation, shortening rate is 8 mm/a and erosion rate is 3×10-7 a-1 above erosion datum plane (equivalent to an erosion of 0.3 mm for each 1000 m per year). When the basement is horizontal and deposited in an average sedimentation rate of 1.6 mm/a with the fillingleveling up sedimentation mode, it is likely that passive-roof duplex similar to the structural style in the Fusha area will form after 3.5 Ma. Whereas a piggyback basin will be formed when laterally varied subsidence velocity and a filling-and-leveling-up sedimentation mode are used in the model，which is similar to the structural style in the Wupoer area. Simulation results indicate syntectonic sedimentation has a huge impact on the FTB: a lot of sediment accumulation in the front of FTB is beneficial to the formation of passive-roof duplex，whereas accumulation at the top and slope of FTB is more conductive to the formation of piggyback basin. Simulation results also show that both Fusha area and Wopoer area have the same filling-and-leveling-up sedimentation mode, but different subsidence of basement has resulted to different depositing. In a small area from hinterland to foreland with an area of less than 80 km, the basement of the Fusha area was dominated by horizontal subsidence, with the foreland FTB deposited by a large volume of sediments. The basement of the Wupoer area was curved under the compression condition, resulting in that the hinterland subsided greatly and thus accumulating much sediment.