Circulating fluidized bed combustion (CFBC) ash, a by-product of fluidized bed coal-fired sulfur fixation technology, presents an opportunity for recycling and reuse when employed as a supplementary cementitious material in cement composite systems, thereby alleviating environmental pressure. Meanwhile, the rheological characteristics of cement pastes are crucial for optimizing its workability, facilitating diverse engineering applications such as pumping, formwork pressure calculation, and 3D printing. Against this backdrop, this study systematically explores the impact of CFBC ash, varying in particle size and content, on the rheological properties of Portland cement (PC) paste. Findings reveal that elevated CFBC content correlates with heightened yield stress and viscosity of the paste. Furthermore, incorporating CFBC with finer particle size distribution amplifies these rheological parameters. Thixotropy mirrors the alterations in dynamic yield stress and viscosity, indicating that CFBC ash addition enhances paste thixotropy. In PC-CFBC ash composites, G’ values consistently surpass G’’, suggesting early-stage elasticity during oscillation testing. Thixotropy in PC-CFBC ash composites is intricately linked to superplasticizer adsorption capacity, while viscoelastic evolution of the paste is governed by hydration kinetics.