To improve the working performance of ultra-high performance concrete (UHPC), two kinds of water reducers with different structures, i.e., polyoxyethylene (EO) polyether (PCE-1) and propylene oxide block polyoxyethylene (EO/PO) polyether (PCE-2), were prepared. The performance of cement-silica fume slurry in a low water-binder ratio with two water reducing agents was evaluated via slurry fluidity, slurry viscosity test, total organic carbon test, and surface tension test, and the relevant mechanism was discussed. The results show that at PCE-2 in the slurry at 0.16 of water-binder ratio and 5%-25% of silica fume content (in mass fraction) has an adsorption-dispersion effect on silica fume rather than PCE-1, thus increasing the fluidity of cement-silica fume slurry. The block polyether structure of water reducer can adjust the surface tension and viscosity of the interstitial fluid, and reduce the viscosity of the cement-silica fume slurry.