Abstract: To address the problem of floor heave instability in weakly cemented soft rock roadways under the coupled effects of high stress and water−rock interaction, where traditional single support methods are difficult to maintain long-term stability, the return airway of the 11303 working face in Hongqingliang Coal Mine was selected as the engineering background. A coordinated support concept of “anchoring-grouting-sealing” was proposed and abstracted into a layered “cementation-sealing” composite structure. Based on the particle discrete element theory, composite models with different layered proportions were established using PFC^3D. The Linear Parallel Bond Model (LPBM) was adopted to simulate the cementation effect between particles, and the model parameters were calibrated through laboratory uniaxial compression tests. On this basis, uniaxial compression numerical simulations were conducted on composite structures with different layered proportions to analyze their strength characteristics, crack evolution behavior, and mesoscopic particle failure modes. The results show that the bearing capacity of the composite structure is mainly controlled by the grouted cementation layer. As the sealing layer thickness increased from 10 mm to 50 mm, the peak strength of the composite structure increased from 50.99 MPa to 52.72 MPa. However, when the sealing layer thickness further increased to 70−90 mm, the peak strength decreased to 49.79−46.18 MPa, indicating that an appropriate sealing layer can enhance interlayer cooperative bearing capacity, whereas an excessively thick sealing layer weakens the overall mechanical performance. The crack evolution process exhibited characteristics of “initiation−stable propagation−rapid increase before peak failure”, and shear cracks dominated the failure process, accounting for more than 91% of the total cracks. With the increase in sealing layer proportion, the total number of cracks gradually decreased, and the crack distribution evolved from overall dispersion to local concentration. Particle failure analysis indicates that the high-slip zones were mainly concentrated in the middle of the specimen and near the layered interface, and the failure mode gradually transformed from concentrated strong shear failure to dispersed progressive failure. The results provide a theoretical basis for the optimization design of “anchoring−grouting−sealing” composite support structures and the stability control of weakly cemented soft rock roadway floors.