The spatial variability of soil mechanics parameters poses challenges for engineering construction. Based on a metro tunnel project in Xi'an, this paper explores the impact of spatial variability on surface deformation caused by shield tunnel construction. It considers the elastic modulus E, cohesion c, and internal friction angle φ of loess. A random field model of soil mechanics parameters is established using covariance matrix decomposition, combined with the Monte Carlo method, to analyze the influence of parameter variability on surface deformation through finite element analysis. The findings indicate that: Considering spatial variability can reduce risks associated with using fixed values as monitoring controls. Spatial variability significantly affects the dispersion and envelope range of surface deformation curves, but does not change the deformation trends and patterns. The variability of the internal friction angle φ has the most substantial impact on surface deformation, followed by elastic modulus E and cohesion c, which have similar effects. Correlation distance and coefficient of variation are positively correlated with surface deformation. Greater correlation distance and coefficient of variation lead to more dispersed surface deformation results, increasing the maximum deformation value and exceedance probability. The research suggests that during the investigation phase, efforts should be made to determine the spatial variability characteristics of soil mechanics parameters. During construction, it is crucial to verify site conditions and actively implement engineering measures to ensure surface deformation remains within acceptable limits.