<p>The Xiyu conglomerate is widely distributed in the north and south of Tianshan Mountain in Xinjiang and has softening characteristics in contact with water. Based on the uniaxial compression test of calcareous cemented Xiyu conglomerate, this paper establishes a particle flow fine-scale model to study the macroscopic mechanical response of rock samples in uniaxial compression test under water immersion and explores the distribution characteristics and evolution law of contact force, cracks and other fine-scale structural parameters of rock samples in the process of the test. The results show that the stress–strain curves of calcareous cemented Xiyu conglomerate samples under water immersion are basically the same, including compression, elasticity, yielding and destruction stages; with the increase in water immersion time, the uniaxial compressive strength and elasticity modulus show a deterioration trend in general, of which the deterioration effect is especially obvious when immersed in water for 7 d. The uniaxial compressive strength and modulus of elasticity show an overall deterioration trend when immersed in water for 7 d, which is the most obvious. The specimens obtained by PFC numerical simulation can represent the macroscopic mechanical properties of calcareous cemented Xiyu conglomerate, and the water immersion effect leads to the gradual deterioration and reduction of the bond strength between particles within the specimen, the non-uniformity is enhanced, and the overall bearing capacity is gradually reduced, and the damage mode is gradually changed from the tensile damage to the tension-shear mixing damage; With the gradual increase of the immersion time, the sudden change points of the number of three types of cracks, namely, total cracks, tensile cracks and shear cracks, within the rock samples corresponded to the damage characteristic points of the macroscopic stress-strain curves, with the tensile cracks always dominating, and the proportion of shear cracks gradually increasing. With the increase in axial load, the number of contact force gradually changes from spherical to spindle-shaped distribution before loading, and its direction is 90° to the loading direction; the number of contact force decreases after the peak, and macroscopically, it is manifested as the sharp decrease in rock bearing capacity and final destruction. The conclusions of the study can provide a reference for the analysis and control of engineering deformation and stability related to the Xiyu conglomerate.</p>