In order to study the influence of deep foundation pit excavation on the mechanical behaviour of adjacent underground pipelines, this paper takes a subsidiary foundation pit project of Nanjing Metro as an example, considers the complex conditions in the process of foundation pit excavation, and adopts numerical simulation methods to analyse the deformation law of underground pipelines in different situations. The results show that the vertical displacement of the underground pipeline presents a symmetrical distribution form, and reaches the peak value on the symmetrical surface of the pit excavation; it reveals the influence law of the pipeline diameter, the pipeline material, and the horizontal distance of the pipeline from the edge of the pit on the mechanical behaviour of the pipeline. Through the in-depth study of these key parameters, it can effectively guide the pipeline protection and monitoring work in the actual project, and guarantee the safety and stability of underground pipelines.