Objectives: The Southwest region of Tarim basin is characterized by complex structures and contains rich gas resources, but fine tectonic research is just beginning, requiring detailed geophysical investigations. Methods: We have been carried out surface gravity and magnetotelluric investigations in 2012~2015, obtaining new geophysical data, which contain information of structures. The wavelet transformation and gravity inversion are used for threedimensional density perturbation imaging of the upper crust layers, which produce density perturbation maps on planes at depths 2.2 km , 7.4 km and 10.8 km respectively, enabling us to insight the deep basin from viewpoint of density properties.  Results: The inversed 3D density perturbation model indicates that the studied region belongs to a special structure type called the upthrust nappe structure wedge, which contains several uplift and depression zones. From the 3D density perturbation maps one can see the vertical variation of the uplift and depression zones. Comparing with the 3D electrical resistivity maps that are inversed from magnetotelluric investigation data, we find that the low density anomalies are well coincided with the low electrical resistivity anomalies, while the high density anomalies are well correlated with the high electrical resistivity anomalies spatially, supporting the results of the 3D density perturbation imaging.  Conclusion: The results of the 3D density perturbation imaging reveal the details of the upthrust nappe structure wedge in the southwest Tarim basin, which was caused by the Cenozoic collision between the India subcontinent and the Eurasia plate as a longdistance effect.