The flexural mechanical behavior of ultra-high performance concrete （UHPC） at different temperatures was studied. The brittle failure evolution mechanism of UHPC was characterized with energy absorption efficiency， acoustic emission energy release rate and ring-down count. Three flexural constitutive models of UHPC were established. The results show that when the temperaure is -170 ℃， the first-peak and maximum flexural strength of UHPC increase from 10.6， 20.9 MPa at 20 ℃ to 21.1， 42.5 MPa， respectively. Within the range of 20--170 ℃， with the decrease of temperature， the brittleness of UHPC firstly increases and then decreases， showing the minimum brittleness at -80 ℃ and the maximum brittleness at -170 ℃， respectively. Compared to the constitutive models of Prof. Guo Zhenhai and Dr.Wu Zemei， the linear-nonlinear constitutive model has the highest prediction accuracy and the lowest data variability.