In order to consider the spatial variability of geotechnical parameters and the determinism of the borehole data obtained from the preliminary geological survey, and improve the accuracy of deformation risk assessment, a method of braced excavation deformation probability assessment based on constrained random field was proposed. Kriging theory and random field theory were used to generate a constrained random field considering the borehole data. Numerical analysis and Monte-Carlo method were then adopted to establish a probabilistic assessment model, and analyzed the influence pattern of parameter spatial variability on the environmental effects and enclosure structure deformation caused by braced excavation. Compared with the traditional deterministic analysis and the probabilistic analysis based on the traditional random field, the results show that the probabilistic analysis method based on the constrained random field can avoid underestimating the risk by the traditional deterministic analysis and overestimating the risk by the traditional random field method, so as to evaluate the risk more reasonably. The research results can provide useful references for further development of countermeasures for braced excavation risks.