Abstract: Synthesis utilization of oil flow visualization, PIV experiment and CFD simulation is applied on turbulent juncture 3-D separation flows to study the features and explanations for different structures in the surface oil flow patterns. This focused on the connection between the space and surface structures. The result shows that two distinct oil lines caused by the strong separation are both 3-D separation lines, which the primary separation line is upstream while the secondary separation line downstream. Surface oil flow pattern is time-averaged result of the unsteady flow, and a four vortex structure system dominants the mean flow. The primary separation line is corresponding to Lighthill 3-D separation model, and the secondary one is based on Maskell 3-D separation model. The secondary separation line has more oil accumulation compared with the primary one and shows lower shear stress, because the higher vorticity interaction of primary vortices and secondary vortices lead to higher shear effects on the surface. As a result, the secondary separation line can also be recognized as low shear stress line. The both sides of low shear stress line are the regions of high shear stress caused by the primary vortices and secondary vortices.