In order to study the compressive properties, deformation rules and improvement mechanism of sand improved by the organic polymer composite fiber, through the unconfined compressive strength test and the numerical simulation, the laws of force chain change and microcrack development in the compressive deformation process of improved sand were analyzed, from which the destruction and improvement mechanism of improved sand were summarized. The results show that the organic polymer composite fiber can effectively improve the compressive strength of sand. With the increase of organic polymer content, the unconfined compressive strength of improved sand increases. In addition, with the increase of organic polymer content and strain, the force chain evolution and microcrack development of the improved sand change obviously. The force chain changes from a ring-column structure to an arch structure, and the total number of microcracks and tensile microcracks also increases, indicating that failure changes from single path to multiple paths. The improvement mechanism shows that the organic polymer composite fiber can effectively improve the compressive properties of sand by binding and wrapping the sand grains. When the load is too large, the organic polymer film breaks, the fibers break and slip gradually, and the network structure becomes unstable gradually. In such a condition, the sand is forced to displace and rotate, forming local micro-cracks that eventually develop and extend to form cracks resulting in damages.