Abstract: This paper aims to study the distribution of forces along the prestressed anchor rod during the excavation of foundation pits. A filed experiment is carried out to investigate mechanical behavior of anchor rod through a comprehensive reconstruction project at Xining Railway Station. The temporal and spatial distribution of axial forces and frictional forces is obtained. The development of potential slip plane is analyzed, and the relation between free part and anchored part of the prestressed anchor rod is discussed. The results show that, if unanchored, the initial stress of the prestressed anchor rod will be increased which is unfavorable. If anchored, the shear stress decreases steadily along the anchored part and then transports backward, but the maximum shear stress doesn’t change. The potential slip plane of slope moves deeper inward due to the effects of anchor rod. At the same time, the exit of the slip plane moves upwards from the foot of the slope, and locates at about one third the height from the bottom of the pit. On one hand, the prestressed anchor rod with a longer free length has better mechanical performance, however the one with a certain length that is enough to prevent the potential slip of slope is more economical and reasonable. On the other hand, the anchored length of the anchor rod, which provides sufficient anchoring force, is better to be short so that the anchor is economical and efficient.