Abstract: In order to explore the influence of green coir fiber on the fundamental properties of ultra-high performance concrete (UHPC), this paper performed experimental research on the effects of fiber content and length on the expansion behavior, compressive strength, elastic modulus, and flexural strength of UHPC. Additionally, the typical failure characteristics of UHPC reinforced by coir fibers were also analyzed by using scanning electron microscopy. The experimental results indicate that the workability and compressive strength of UHPC significantly decrease with the content and length of coir fiber increase, while the performance of UHPC can meet requirements within acceptable thresholds. On the other hand, adding coir fibers significantly improves the flexural strength and ductility of UHPC, and a reasonable upper limit exists for both fiber dosage and length. Compared with other plant-based fibers, coir fiber demonstrates a more pronounced capacity to improve the flexural performance of UHPC.