The effect of geometric characteristics of castings on the linear contraction of sand casting Mg-9Gd-3Y- 0.5Zr alloy (VW93) was studied. The dimensions of free and restrained structure castings were precisely extracted by a 3D scanner and then used for the calculation of the contraction coefficient. The ratio of sand core volume to enveloping volume of casting (γ), was introduced to quantify the degree of constraint caused by sand cores. According to the statistical distribution of free contraction coefficients and the evolution of restrained contraction coefficients against γ, it is indicated that the free contraction coefficient of VW93 alloy equals 1.96%, and the restrained contraction coefficient declines linearly with the increasing γ, which supplies support on predicting the contraction of VW93 alloy through the geometric characteristic of castings.