The effect of Astragalus polysaccharide (APS) on the formation and physicochemical properties of whey protein isolate (WPI) gel was systematically evaluated. It was found that visual appearance, gel strength, water-holding capacity, and swelling ratio of the APS-WPI gels were enhanced with increasing APS concentration. APS addition also notably promoted the enhancement in the absolute value of zeta potential with a simultaneous increase in size. Rheological results exhibited that APS-WPI gels had higher apparent viscosities, loss modulus, and storage modulus than native WPI. Scanning electron microscopy demonstrated that the water cavity structure of the gel network was denser when 1.2 g/100 mL APS was used. Additionally, disulfide bonds and hydrogen-bonding interactions between WPI and APS were considered the major contributing forces affecting the formation of APS-WPI gels, as confirmed by Fourier transform infrared spectroscopy and molecular forces. These results indicated that APS is effective in improving the gelling properties of WPI, which provides a reference for the potential application of a gelling agent in functional foods.