The use of plant-derived emulsified gel systems as fat substitutes for meat products has always been an important direction in the development of healthy foods. In this study, a composite matrix emulsion gel was prepared with soy protein isolate (SPI) and different concentrations of cassia bean gum (CG), and then the selected emulsion gel was applied to meat sausage as a fat substitute to explore its stability. Our results showed that the hardness, chewiness, viscosity, shear stress, and G' and G″ moduli of the emulsion gel increased considerably with the cassia bean gum concentration, the thickness of the emulsion gel increased, and the pore size decreased. The gel strength of the 1.75% CG/SPI emulsion gel was the highest, which was 586.91 g. The elasticity was 0.94 mm, the masticability was 452.94 mJ, and the water-holding capacity (WHC) was 98.45%. Then, the 1.75% CG/SPI emulsion gel obtained via screening was applied as a fat substitute in meat sausage. With an increase in the substitution amount, the cooking loss, emulsification stability, pH, color difference, texture, and antioxidant activity of the meat sausage before and after freezing and thawing increased first and then decreased. The indexes of meat sausage with 50% fat replacement were not considerably different from those of full-fat meat sausage. This study can provide a theoretical basis for the application of plant-derived emulsified gel systems as fat substitutes in meat sausage.