The influence of long-term solution treatment for various intervals on the microstructure, mechanical properties, and corrosion resistance of the as-cast Mg-5Zn-1. 5Y alloy was investigated. Variation of secondary phases was studied during solution treatment through thermal analysis test and thermodynamic calculations. Tensile and hardness tests, as well as polarization and immersion tests, were performed to evaluate the mechanical properties and corrosion behavior of the ascast and heat-treated alloy, respectively. Results show that solution treatment transforms I-phase (Mg3Zn6Y) into W-phase (Mg3Zn3Y2), as well as dissolves it into the α-Mg matrix to some extent; therefore, the amount of W-phase increases. Moreover, prolonged solution treatment decreases the volume fraction of the phases. In the first stage of solution treatment for 14 h, the tensile properties significantly increase due to the incomplete phase transformation. Although long-term solution treatment sharply decreases the tensile and hardness properties of the alloy, it improves the corrosion resistance due to the transformation of I-phase into W-phase. In fact, it decreases corrosion potential and simultaneously dissolves intermetallic compounds into the α-Mg matrix, resulting in the reduction in galvanic microcells between the matrix and compounds. It is found that the optimum time for long-term solution treatment is 14 h, which improves both corrosion behavior and mechanical properties.