Abstract: Ultra high performance concrete (UHPC) has been widely studied and applied due to its excellent mechanical properties and durability. However, the effect of polyacrylamide flocculant (PAM) content in quartz sand on the performance of UHPC is not yet clear. This article aims to investigate the effect of PAM content on the performance of UHPC and reveal its mechanism of action, providing theoretical support for the optimization design and practical application of UHPC. This article investigates the effect of PAM content in quartz sand on the performance of UHPC through experiments. The flowability of UHPC was evaluated using an expansion test, and its compressive and flexural strength at 7 days and 28 days were tested to assess its mechanical properties. Meanwhile, the influence of PAM content on the internal pore structure changes of UHPC was analyzed. The research results indicate that when the PAM content in quartz sand is within the range of 0% to 0.04%, the expansion degree of UHPC gradually decreases with the increase of PAM content, and the loss of expansion degree increases over time. At the same time, the 7 day and 28 day compressive and flexural strengths of UHPC showed a decreasing trend. Through FSEM and MIP analysis, it was found that the addition of PAM significantly changes the internal pore structure of UHPC, increases porosity, and increases the proportion of harmful pores (>200 nm) and harmful pores (50 nm~200 nm), leading to a decrease in UHPC strength. The PAM content in quartz sand has a significant impact on the performance of UHPC. The addition of PAM will reduce the fluidity and mechanical strength of UHPC, mainly because PAM changes the pore structure inside UHPC, increasing the proportion of porosity and harmful pores. Therefore, the content of PAM in quartz sand should be strictly controlled in the preparation of UHPC to ensure its high performance.