This study systematically engineers a series of Mg-1.0Zn-xCa-ySn alloys (ZXT-series) by systematically varying Sn/Ca atomic ratios (1:4 in ZXT4, 1:8 in ZXT8, and 1:16 in ZXT16) to investigate its impact on microstructural evolution, mechanical properties, and corrosion behavior in Hank's solution. Through systematic investigation, the complex effects of the Sn/Ca atomic ratio on the microstructural evolution were elucidated, and the strengthening mechanisms of the ZXT series alloys were quantitatively analyzed and compared. Consequently, due to the finest recrystallized grains, ZXT16 alloy achieved a peak yield strength of 264 MPa. Notably, the ZXT4 alloy (Sn/Ca ratio 1:4) demonstrated the most balanced overall performance, with a yield strength of ~233 MPa, elongation ~18.0% and a low degradation rate of ~ 0.208 mm/year. This enhanced corrosion resistance in ZXT4 is attributed to its refined grain structure, minimized micro-galvanic activity from CaMgSn precipitates (Volta potential difference ΔVPD ≈ 195 mV with the matrix), and the formation of a dense, stratified, multi-component protective corrosion product layer composed of a Zn-rich (ZnO/Zn(OH)₂) inner barrier, an MgO/Mg(OH)₂/SnO/SnO₂ intermediate matrix, and an outer Ca₃(PO₄)₂ deposition via accelerated nucleation kinetics. A triphasic corrosion progression mechanism further elucidated these observations. This work establishes crucial processing-microstructure-property-corrosion relationships and presents strategic Sn/Ca ratio optimization as a robust pathway for developing advanced Mg-Zn-Ca-Sn biomaterials.