Abstract: The deep foundation pit constructed in alluvial thick clay layer is almost supported by using anchor-pile supporting structure. Due to the loss of prestress caused by anchor deformation and creep of anchorage body and surrounding soils with time after the anchor being locked, the axial force of anchor cables in the foundation pit is unable to be kept at the design level. So the safety of the foundation pit is reduced. To solve this issue, the super tension and secondary compensation tension methods to increase the anchorage force are generally adopted at the construction site. However, it is difficult to determine the value of anchorage force in the above two methods. A simple method is given to calculate the axial force of the anchor applied in the deep foundation pit located in the area of thick alluvial clay in western Jinan. In this method, the deformation of anchorage system is separated into two parts according to the construction situation of the anchors: one part is before the anchor being locked and the other is after being locked. The second part induces the loss of prestress which can be represented by the deformation of free segment of the anchor cables. It consists of the creeps in the anchorage body and the surrounding soils. The results applicable to two construction methods give theoretical and technical assistance for in-situ construction. Furthermore, field monitoring and Plaxis numerical simulation are used to verify the effectiveness of the proposed method. Finally, the applicability of the proposed method is given.