To reveal the performance evolution and degradation mechanism of ultra-high performance concrete(UHPC) under extreme temperature environments, the tensile behavior of UHPC in the range of - 170 ℃ to 200 ℃ was investigated, and the temperature-variation damage and crack evolution of UHPC were in-situ monitored by acoustic emission ( AE ) technology. The results indicate that under the alternating cryogenic and elevated environments, the tensile failure of UHPC with straight and hooked-end steel fibers exhibits strain-hardening characteristics. In first temperature-variation cycle, the mechanical properties of UHPC show fluctuations, with the highest at cryogenic temperatures and the lowest at elevated temperatures. After multiple cycling, the mechanical properties of UHPC decrease accordingly. The AE signal generated by UHPC damage contains abundant failure characteristic parameters, and its AE peak frequency morphology differentiates at cryogenic and elevated temperatures, while that exhibits a centralized single peak distribution in other environments. Except for cryogenic temperatures, UHPC cracking in other environments is mainly tensile cracks, and the number of tensile cracks in the hooked-end fiber group is lower than that in the straight fiber group. As the cycling increasing, the number of tensile cracks in UHPC decreases.