Abstract: In the process of constructional backfilling coal mining (CBCM), the demand for the strength of cemented gangue backfill (CGB) samples varies with different backfill rates. It is planned to adjust the strength of CGB samples by changing the proportion of fly ash in the dosage of cement-fly ash binary cementitious material (hereinafter referred to as the proportion of fly ash), and conduct uniaxial compression tests on CGB samples with different proportions of fly ash (0−90%) to explore the range of strength control of CGB samples, and analyze the failure morphology of the samples using digital image correlation technology, and analyze the brittle ductile failure mode of the samples using AE events. The strength of coal gangue and the strength of cement-fly ash binary cementitious material with different fly ash proportions are measured using the pressure cylinder method and uniaxial compression test, respectively, and the mechanism of strength formation of CGB samples is revealed based on the relationship between the strength of coal gangue and the strength of cement-fly ash binary cementitious material. The research results indicate that the strength of CGB decreases with the increase of proportion of fly ash, achieving a larger range of strength control for CGB: 1.77−30.04 MPa. The failure morphology and the brittle ductile failure mode of samples with different replacement rates of CGB samples are different, and the corresponding strength range of the samples is also different, Material properties have undergone fundamental changes. As the proportion of fly ash increases, in terms of the failure morphology, CGB samples transition from shear failure (0−50% proportion of fly ash) to tensile shear composite failure (60%−80% proportion of fly ash) and then to tensile failure (90% proportion of fly ash), corresponding to the strength ranges are 20.8−30.04 MPa, 5.63−12.35 MPa, and 1.77 MPa; In terms of the brittle ductile failure mode, the samples transition from brittle-ductile failure (0−40% proportion of fly ash) to ductile failure (50%−90% proportion of fly ash), corresponding to the strength ranges are 24.64−30.04 MPa and 1.77−20.8 MPa. The strength of CGB is determined by the strength of the cement-fly ash binary cementitious material (1.43−55.5 MPa) and the strength of coal gangue (with an average strength of 4.43 MPa). Cement-fly ash binary cementitious material contributes significantly to the strength of CGB, while coal gangue limits the strength of CGB. The obtained experimental results provide a certain reference for the strength selection of filling materials in CBCM.