Uniaxial compression tests and acoustic emission (AE) monitoring were carried out on ultra-high performance concrete (UHPC) with different steel fiber volume fractions. The characteristics of AE signals, damage evolution law and stress–strain relationships of UHPC under the uniaxial compression were analyzed. Results showed that the compressive strength of UHPC considerably improved with the increase of steel fiber volume fractions. The evolution law of AE parameters (cumulative counts and energy) had a remarkable correlation with the stress-strain relationship of UHPC during the loading process. Based on the AE data analysis, the cumulative counts’ ratio was used to construct the damage factor. Furthermore, the damage evolution law of UHPC under uniaxial compression was investigated, and its distribution function was discussed. Then combined with damage theory, the UHPC uniaxial compression constitutive equation was obtained.