The rheological properties of the propellant slurry and droplet dynamics during the casting process play a critical role in the manufacturing of propellants. This study systematically investigated the rheological characteristics of a hydroxyl-terminated polybutadiene (HTPB)-based composite solid propellant slurry and optimized the vacuum casting process through integrated experimental and numerical simulation approaches. Rheological tests revealed shear-thinning behavior, with the Herschel–Bulkley model parameters confirming non-Newtonian fluid characteristics. The addition of coarse ammonium perchlorate (AP) particles caused the slurry viscosity to peak at 41 Pa·s, stabilizing at 17 Pa·s after mechanical mixing. A real-time monitoring system combining machine vision and Kalman filtering demonstrated that increasing the vacuum pressure from 10 to 14 psi enhanced the casting efficiency but reduced the dynamic range of droplet impact forces. Numerical simulations validated experimental data, with a casting time deviation of 6.16%–11.07%. The study confirmed that vacuum pressure regulation optimizes process parameters by balancing casting efficiency and impact forces, providing critical theoretical support for propellant defect control and casting process optimization.