The horizontal and vertical components of large oblique thrusts are important parameters in determining the kinematics of earthquake ruptures and regional stress orientation, but their values are mainly based on field measurement of offsets using marker lines. Choosing proper marker lines will directly affect judgment and measurement of the nature of slip fault. Based on our field observation during mapping the 5·12 Wenchuan earthquake surface rupture, we analyzed the features of displacement in the coseismic thrust fault and found that when the marker lines are cross the fault, thrust motion accompanying horizontal shortening results in the oblique displacement. Based on the modes for different scrap this study also gives an equation to calculate oblique slip displacement, and calculated and corrected the displacements in site. Obliquity correction results show no slip motion in the Anxian—Guanxian fault (Pengguan fault), suggesting a pure thrust. Two marker lines (Xiaoyudong fault and Pengguan fault) about a few meters apart are characterized by leftlateral and rightlateral slip respectively, further implying that the main factor affecting displacement error is horizontal shorting but not local change of fault direction. This study indicates that when measuring displacement of thrust faults on site, those marker lines which are perpendicular to fault or mark the angle between marker line and fault should be selected. Only after correction can this measurement stand for the real slip displacement of faults.