Abstract: This study investigates the effects of fiber content, fiber length, and alkali treatment conditions on the flexural performance of coir fiber-reinforced epoxy resin composites. The bending properties were evaluated using three-point bending tests, and the bonding interface was observed through scanning electron microscopy. The results indicate that: 1) as fiber content or length increases, the flexural strength and modulus of the composites initially rise and then decrease, with the peak values observed at fiber length of approximately 10 mm and fiber content of around 3%; 2) alkali treatment significantly enhances the mechanical properties of the composites, with fibers treated in 5% NaOH solution for 6 h performing notably better than those treated in 10% NaOH solution for 3 h. Optimizing the fiber length, content, and alkali treatment conditions can improve the flexural strength of the composite material.