Coal gasification fine ash (CGFA) is a large amount of solid waste produced by the coal chemical industry and has a broad prospect of recycling in building materials. This paper aims to investigate the effect of CGFA on the flowability, rheological behavior, hydration properties, and sustainability of blended cement. CGFA contains mineral components similar to fly ash (FA) and has morphological and physical filling effects that promote the flow of cement pastes. However, water absorption by the residual carbon component in CGFA resulted in severe deterioration of the rheological properties and flowability of the paste. As levels of CGFA replacement increase, the blended cement shows a pattern of decreasing early hydration rate, reducing flowability, and a significant increase in dynamic yield stress. The characteristic hydration reaction peak indicating the transformation of ettringite (AFt) to kuzelite (AFm) was little observed in the mixed samples of CGFA and cement. This phenomenon is attributed to the low content of reactive minerals (mainly aluminates) in CGFA. The residual carbon in CGFA almost did not participate in the hydration reaction of the blended cement system. It also limited the accumulation of hydration products, significantly reducing the strength of the mortar. Considering multiple factors such as cost, carbon emission, embodied energy, and cement consumption, the combined sustainability assessment of the (15%CGFA + 15%FA) combination replacing cement is 14.5% higher than that of pure cement mortar. This study can provide theoretical and technical support for the sustainable development of the coal chemical industry and the construction industry.