The demand for plant protein is increasing significantly due to the shortage of protein resources. Walnut protein, the main by-product of preparing walnut oil, has limited application in the food industry due to its poor solubility. It was found that the soy protein isolate (SPI) concentration had significant effects on the gel properties of the walnut protein isolate (WNPI)-κ-Carrageenan (KC) composite system treated with 15 mmol/L NaCl. The results showed that the gel strength of the composite system increased first and then decreased with the increased concentration of SPI from 0 to 2.5%. The best rheological properties, texture properties, water holding capacity ((92.03 ± 1.05)%), swelling ratio ((2.04 ± 0.19)%), freeze-thaw stability and thermal stability (85.53 °C) of the composite gel were found at an SPI concentration of 1%. In the meantime, the secondary structure of protein had the least α-helix content of 10.17% and the highest β-sheet content of 39.64%, the fluorescence intensity and free sulfhydryl content reached the highest value. 1% SPI could also act as a filler for WNPI to enhance the intermolecular forces such as hydrophobic interaction between the two substances, thus forming a stable gel network structure. This study can provide technical support for improving the gel properties of walnut protein and producing new plant protein gel products.