The rheological properties of propellant slurry and the droplet dynamics during casting play a vital role in propellant manufacturing. Firstly, this study systematically studied the rheological properties of hydroxyl-terminated polybutadiene (HTPB)-based composite solid propellant slurry. Secondly, the vacuum casting process was optimized by an integrated experimental method. The real-time monitoring system combined with machine vision and Kalman filtering showed that increasing the vacuum pressure from 10 psi to 14 psi can improve the casting efficiency, but it will reduce the dynamic range of the droplet impact force. Finally, the numerical simulation verified the experimental data, and the casting time error was 6.6%–11.1%. The study proved that vacuum pressure regulation can optimize process parameters by balancing casting efficiency and impact force, providing important theoretical support for propellant defect control and casting process optimization. In addition, the droplet acceleration is consistent with the air resistance theory, which verifies the reliability of the equivalent mass model. These findings provide a multi-dimensional basis for improving the casting process of propellants with complex geometries and ensuring quality stability.