The effect of the process aid “OPS” on the rheological properties of hydroxyl-terminated polybutadiene propellant was investigated by formulating different components of high-solid-content slurry, and the change in slurry viscosity with shear rate, surface morphology of solid-phase particles, and contact angle of the relevant interfaces were characterized. The results showed that the polyalkene polyamine surfactant OPS could significantly reduce the apparent viscosity and enhance the rheological properties of the slurry, to up to a 30% reduction, and the effect was achieved by adjusting the interfacial properties of the aluminum powder and the binder system. With the addition of 0.1% OPS, the contact angle of the interface between the aluminum powder and the binder was obviously reduced, from 97° to 30°, and the wetting was significantly enhanced, so it was judged that the OPS was suitable for HTPB-based composite propellants.