Ultra-high performance concrete ( UHPC) has wide application prospect in engineering field because of its excellent performance. However, due to large shrinkage, the further promotion and development of UHPC were limited. Magnesium oxide (MgO) expansion agent has the characteristics of stable expansion, controllable activity, and is suitable for UHPC, so it is regarded as a UHPC expansion agent with great potential. The purpose of this study was to investigate the effects of different activity and dosages of MgO expansion agent on the performance of UHPC, and to further explain the mechanism of different activity of MgO expansion agent on the performance of UHPC by means of XRD and SEM. The results show that in UHPC mixed with MgO expansion agent, shear thickening is observed at low speed, while shear thinning is observed at high speed. Compared with cement particles, MgO expansion agent particles have stronger adsorption capacity to water. Therefore, the higher the activity of MgO expansion agent is, the more significant the decrease in the fluidity of UHPC is. And the dynamic yield stress of UHPC increases, which requires more pumping energy in practical engineering.