A series of explosive thermal spalling tests at elevated temperature and compression tests after high temperature were conducted on coir fiber reinforced ultra-high performance concrete （UHPC） by using an electric furnace in this study， aiming to shed light on the influence of the length and volume dosage of coir fiber on the fire resistance of UHPC exposed to different high temperatures. The results indicate that incorporating 1% of coir fibers with a length of 2 cm in UHPC can effectively suppress the high-temperature spalling. Microscope analysis reveals that a tangential pore network is generated between coir fibers and UHPC matrix due to shrinkage， decomposition， and carbonization of coir fibers at high temperature， which provides channels for the escape of internal water vapor as well as improving the thermal spalling resistance of UHPC. At 200 ℃， 400°C， 600 ℃， and 800 ℃， compressive strength and elastic modulus of UHPC， incorporating 1% of coir fibers with 2 cm long， significantly decrease with an increase in control temperature. Also， within the range of the control temperature， the ratio of the axial compressive strength to the cubic compressive strength varies between 0.62 and 0.92. On the other hand， compressive strength and elastic modulus decrease with the increasing volume dosage of coir fiber under the same control temperature， and the ratio of uniaxial compressive strength and cubic compressive strength varies with control temperatures. In terms of the improvement effect on fire resistance of UHPC， coir fiber is superior to steel fiber and is comparable to jute and flax fibers. Based on the test data and the existing formula， a special formula is proposed for evaluating the residual compressive strength of coir fiber reinforced UHPC exposed to high temperatures.