This study aimed to address the problem of high viscosity and poor workability of high-strength and ultra-high-strength concrete mixtures caused by high usage of binding materials and low water-binder ratios, which in turn affects construction quality, by developing a new type of reducing agent admixture. The reducing agent admixture was formulated using polycarboxylic acid water-reducing agent mother liquor, defoaming agents, air-entraining agents, rheological agents, retarders, and early-strength agents as functional additives. Single-factor experiments and orthogonal experiments were performed to investigate the effects of different components on the viscosity, flowability, and strength of cement paste and mortar, in order to determine the optimal ratio of the reducing agent admixture. The influence of the reducing agent admixture on hydration heat and hydration products of cement was investigated by conducting micro-tests on hydration heat and XRD. The experimental results show that the mass ratio of optimal preparation process for the reducing agent admixture is water reducer mother liquor∶polyether defoaming agent∶AE-II∶triterpenoid saponin∶rheological agent∶sodium tripolyphosphate∶sodium hexametaphosphate∶sodium gluconate∶citric acid∶sodium thiosulfate∶sodium sulfate=1 000∶3.5∶0.5∶1.5∶4∶10∶5∶15∶5∶15∶10. When a viscosity reducing admixture is added to the cement slurry, the heat release rate of cement hydration is delayed and the amount of hydration products is inhibited.