At present, the mechanisms by which exogenous additives regulate hydroxypropyl starch/carrageenan blends remain unclear, and systematic research supporting performance optimization is lacking. This study systematically investigates the effects of two types of exogenous additives—salts and plasticizers—on the properties of these blends, employing infrared spectroscopy, X-ray diffraction, textural analysis, contact angle measurement, and ultraviolet spectroscopy for characterization. The results indicate that salts and plasticizers primarily regulate the structure and properties of the blends through hydrogen bonding. Salts can maintain the blends' amorphous structure, enhance mechanical properties, alter surface hydrophobicity, and affect light transmittance; monovalent salts exhibit mild effects, while divalent salts provide superior regulation, though phase separation tends to occur when the addition exceeds 1%. 0.5% magnesium chloride was determined to be the optimal salt concentration. At this concentration, the blend's hardness increased from 3.92 N to 4.61 N, the elongation at break rose from 25.40% to 48.60%, the contact angle increased from 101.17° to 107.22°, and the light transmittance improved from 85.94% to 88.67%. The effect of plasticizers is concentration-dependent: low concentrations improve mechanical properties and hydrophilicity, while high concentrations reduce hardness and hydrophilicity but continue to increase elongation. The plasticizing efficiency was glycerol > sorbitol > sorbitol sorbitan solution, in that order. With 20% glycerol as the optimal plasticizer, the blend hardness reached 5.22 N, the elongation at break was 58.12%, the contact angle decreased to 88.17°, and the light transmittance was 88.10%. In summary, by clarifying the mechanisms of additive action, a theoretical basis is provided for optimizing the properties of polysaccharide blends.