Frozen Nemipterus virgatus surimi was utilized as the raw material, and surimi gels were prepared by adding different amounts of soy protein isolate (SPI) (0%~5%, m/m) on top of 12% cassava starch to investigate the effect of SPI on the quality of N. virgatus surimi gel. The whiteness, textural properties, water-holding capacity, cooking loss, secondary structure, and microscopic structure of the gel were analyzed. The results indicated that surimi quality improved the most when 3% SPI was added. The strength of the surimi gel peaked at 24.98 N•mm, an increase of 33% compared with that of the control group. Hardness and chewiness also reached their maximum values, increasing by 27.95% and 30.21%, respectively, compared with those of the control group. Water-holding capacity peaked at 90.77%, an increase of 4.91% compared with that of the control group, and cooking loss was the lowest. The gel structure exhibited the best uniformity and compactness. SPI addition may also have affected protein denaturation, increasing hydrophobic interactions and hydrogen bond content in the surimi gel, thereby transforming α-helices into β-pleated sheets and improving the performance of the surimi gel. Therefore, adding a moderate amount of SPI during the production process of N. virgatus surimi gel could improve its quality. This study provides a theoretical basis for the processing and utilization of N. virgatus surimi in surimi products.