Magnesium rare earth alloy castings with varying degrees of contraction constraints were designed by combining rods and flanges with different cross-sectional sizes, and the constrained contraction behavior of Mg−9Gd−3Y−0.5Zr alloy castings was investigated under low-pressure and gravity casting conditions. Precise dimensional measurements were performed using a 3D scanner, and the data obtained were used to analyze trends in constrained contraction coefficient (CC) values from multiple perspectives. Several evaluation factors, including the ratios of flange to rod area and casting to envelope volume, as well as the combination of geometric characteristics and casting modulus, were explored for their effectiveness in assessing the degree of constraint on the castings. The geometric characteristics and casting modulus were integrated through regression analysis to determine optimized evaluation parameters, based on which an equation for correlating free and constrained CC values was established.