Abstract: As foundation pit dewatering cannot immediately achieve the predetermined groundwater level, the adjacent pipeline experiences different deformations due to fluctuations in the groundwater level with time. This paper introduces an innovative approach by integrating the theory of unsteady seepage and the principle of effective stress. The pipeline is simplified as an Euler–Bernoulli beam resting on a Pasternak foundation model. A calculation model is developed to analyze the deformation of a pipeline adjacent to a foundation pit, incorporating the time-dependent effects of dewatering. By solving the deformation control equation through integration, the study reveals the longitudinal deformation characteristics of the pipeline. The accuracy and superiority of the calculation method are verified by comparing the monitoring data, existing model theory and numerical calculations. The results demonstrate that the proposed method can accurately calculate pipeline displacement at any location and over different time periods. The affected range of adjacent pipelines can extend to more than 10 times the drawdown depth. When the permeability coefficient K' decreases from 0.075K to 0.010K, the maximum pipeline displacement remains within the controllable limit of 2 mm. However, when the dewatering time of the foundation pit is less than 3 days, the pipeline experiences a rapid increase in displacement. In such cases, monitoring frequency should be intensified to prevent pipeline damage.