The poor plastic deformation ability of magnesium alloy, resulted from its close-packed hexagonal structure and only two independent basal <a> slip systems at room temperature that cannot meet the von Mises criterion, has extremely restricted its application. As the α-Mg dendrites grow along with the heat flow in directional solidification, the uniform columnar crystal structures obtained in Mg can effectively improve its mechanical properties. And the mechanical properties of the anisotropic magnesium alloys were heavily affected by the orientation controlled by the directional solidification parameters. In this work, the effects of Gd content (3.0%, 4.5%, 6.0%, mass fraction) on the microstructure and mechanical properties of directionally solidified Mg-xGd-0.5Y alloy were investigated. The tensile deformation behavior at room temperature was analyzed by EBSD technique. The results showed that the Mg-xGd-0.5Y alloys have a longitudinal grain boundary parallel to the heat flow direction and a preferential growth along the normal direction of the (\mathrm{11}\stackrel{\text{?}}{\mathrm{2}}\mathrm{0}) plane at a withdrawn rate of 3 mm/min. The cross section of the columnar crystal was triangle or crisscross petal in shape, and the secondary branch gradually changed from three branches of 3.0%Gd to four branches of 6.0%Gd. The Mg-6.0Gd-0.5Y alloy with more columnar crystal growing along with <\mathrm{22}\stackrel{\text{?}}{\mathrm{4}}\mathrm{3}> direction had higher tensile strength (107 MPa) and post-break elongation (32.56%) at room temperature, and its deformation mechanism was basal <a> slipping and {\mathrm{10}\stackrel{\text{?}}{\mathrm{1}}\mathrm{2}} extension twinning. When the crystal growth directions dispersed (concentrated on the <\stackrel{\text{?}}{\mathrm{1}}\mathrm{2}\stackrel{\text{?}}{\mathrm{1}}\mathrm{0}> and <\mathrm{22}\stackrel{\text{?}}{\mathrm{4}}\mathrm{3}>) in the Mg-3.0Gd-0.5Y alloy, it had low post-break elongation (14.88%) because of poor synergistic deformation ability, which have {\mathrm{10}\stackrel{\text{?}}{\mathrm{1}}\mathrm{2}} extension twins and {\mathrm{10}\stackrel{\text{?}}{\mathrm{1}}\mathrm{1}} contraction twins to accommodate strain.