Abstract: The escalating production of industrial solid waste, combined with the dwindling availability of natural resources, has intensified the focus on waste recycling. However, the heterogeneity and complexity of waste pose significant challenges to determining process parameters. In this study, burnt coal cinder (BCC), granite powder (GP), and high-calcium fly ash (Class-C FA) were used as raw materials, and the response surface methodology (RSM) and single-factor experiments were applied to optimize the process parameters for geopolymer preparation. The optimized precursor powder composition was determined to be a mass ratio of 1.6:0.9:7.3 for BCC, GP, and Class-C FA. The NaOH-precursor powder ratio and liquid–solid ratio were adjusted to 0.084 and 0.222, respectively. The curing condition was set at 80°C for 24 h. The resulting 28 d-aged multi-solid wastes-based geopolymer exhibited a high compressive strength of 61.34 MPa. The microstructure, mineral phase, and atomic bonding of geopolymers were investigated using X-ray diffraction (XRD), thermal analysis (TA), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). Findings indicate that the compressive strength of geopolymer is most significantly influenced by the Class-C FA, followed by BCC. Furthermore, a minor addition of GP can optimize the structural density of the geopolymer. The Ca present in the Class-C FA participates in the geopolymerization, forming a hybrid N–(C)–A–S–H gel. RSM optimization facilitates the synergistic utilization of multi-solid wastes, ensuring an even distribution of gel and filler. This research establishes a theoretical framework for optimizing the preparation parameters of multi-solid wastes-based geopolymer and its subsequent applications; it holds significant scientific implications for the circular economy, resource transformation, and environmental conservation.