Establishing the intrinsic correlation between microstructural heterogeneity and mechanical properties is a challenging issue of metallic glasses. The ratchet behavior was examined in a Zr-based metallic glass under cyclic tensile loading well below the yield point, particularly near the glass transition temperature. It is found that strain evolution during cyclic loading shows heightened sensitivity to temperature and stress rate. Also, creep behavior mirrors the ratchet strain induced by cyclic loading. The proposed constitutive model, integrating the Burgers model with defect concentration based on free volume theory, effectively describes strain evolution during cyclic loading near glass transition temperature. Both macroscopic and microscopic perspectives are included in this model. The results verify that metallic glasses exhibit significant viscous characteristics, displaying noticeable creep deformation under low stress rates and amplitudes, which contributes to ratchet behavior. The fitted parameters show that plastic viscosity decreases with temperature and increases with stress rate, corroborating the decrease of tensile yield stress with temperature increasing; also, the fitted relaxation time increases with loading frequency, reflecting evolution of defect concentration. Structural relaxation competes favorably against stress-driven rejuvenation throughout the cyclic process, suggesting potential in tuning metallic glasses properties through innovation thermos-mechanical processing techniques.