When building expressways on soft soil foundation, stability and settlement are two key points in the design. The control of roadbed stability and settlement directly affects the safety and comfort of driving. The article studies the stability of soft soil subgrade through numerical simulation analysis. Understanding the key factors and influencing laws that affect the stability of soft soil subgrade plays a key role in the design of subgrade stability, and also plays a guiding significance for the focus of early investigation and the comparison and selection of expressways built on different soft soil sections. The article simulates the stability changes of slopes under different working conditions, focusing on the distribution laws of cohesive force c, friction angle psi, soft soil layer thickness, burial depth, and the influence of parameters such as groundwater depth on slope stability. The corresponding laws are summarized to provide guidance for the design of soft soil subgrade.