<p>Due to the increased infrastructure construction in loess regions, geological disasters and engineering problems caused by loess disintegration resulting from water immersion have significantly increased. Traditional disintegration tests cannot reveal the boundary effects of loess disintegration, are not repeatable, and parameter identification is challenging. Indoor disintegration tests and scanning electron microscopy were conducted to analyze the dynamic disintegration of loess. The particle flow code (PFC) numerical simulation software was used to examine the influence of the particle shape and size on loess disintegration during water immersion. The results show that the curve describing the loess disintegration rate for different initial moisture contents has an S-shape and can be described by the Gompertz model. Disintegration at the microscopic scale occurs due to the weakening of bonds between different functional units in the loess (aggregates formed by the cementation of particles of different sizes with clay particles and soluble salts) during water immersion. The PFC simulation indicates a decrease in the contact bonds. The contact between the loess particles and water is delayed for large specimens because water penetration requires more time, reducing the disintegration rate. The sharper the edges and the larger the specimen’s curvature, the stronger the boundary constraint and the higher the disintegration rate for specimens with the same immersion area. The results provide a theoretical basis and practical reference for preventing geological disasters and engineering problems related to loess disintegration.</p>