Ili loess is susceptible to substantial collapsible deformation due to water infiltration, leading to various engineering failures. To investigate the characteristics of water infiltration and the mechanisms of collapsible deformation, a field immersion test was conducted in the collapsible loess region of Ili. The changes in water content, water diffusion form, infiltration water volume, and surface collapsible deformation during immersion were analyzed. Numerical simulations explored surface collapsible deformation characteristics under varying saturation infiltration ranges. The results showed that the average vertical diffusion rate in the experimental area was 0.38 m/d, while the average radial diffusion rate was 0.17 m/d. Over time, the water diffusion pattern transitioned from elliptical to conical, with a wet front angle of 41 and a saturated front angle of 20. The quantitative analysis of the vertical and radial water diffusion rates and infiltration water volume of Ili loess over time was conducted. A mathematical relationship between infiltration range and cumulative infiltration volume per unit area was derived. The correction coefficient for collapsible in the experimental area was determined to be 0.74, exceeding the recommended value of 0.5 in the specifications. The surface collapsible deformation correlates with water infiltration and can be divided into four stages: stable immersion, severe collapse, slow collapse, and consolidation settlement. As the saturation front angle increases, the surface collapsible deformation value maintains good consistency with the calculation results of the subsidence trough formula. The variation in the width of the subsidence trough aligns with the influence range of surface collapsibility, both following an exponential increase.