Maraging steel is indispensable in aerospace, automotive, and other fields because of its excellent balance of strength and toughness. However, the Bauschinger effect, which is commonly induced by cyclic loading during service, can significantly alter a material's yield strength. This leads to a loss of dimensional accuracy and even premature fatigue failure, thereby compromising service reliability. Current research still lacks a systematic understanding of how microstructural heterogeneity regulates the Bauschinger effect. By analysing the cyclic deformation behavior of 18Ni(300) steel under solution-treated (ST) and solution-treated plus aged (STA) conditions, combined with characterization viaelectron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), this study reveals that the synergistic effect of nonuniform martensite block size distribution and localized enrichment of geometrically necessary dislocations (GND) is the key mechanism exacerbating the Bauschinger effect. Research has demonstrated that homogenizing a material's microstructure and GND density through appropriate heat treatment protocols is an effective approach to mitigate the Bauschinger effect.