Maraging stainless steel has extensive applications in aerospace, marine, and other demanding fields because of its high strength. However, its susceptibility to pitting corrosion in Cl^--containing environments considerably limits its practical applications under corrosive conditions. This study investigates the effects of three solution treatments—high temperature solution (HS), low temperature solution (LS), and cyclic low temperature solution (CLS)—on grain size and reversed austenite formation in Cr-Ni-Co-Mo maraging stainless steel. Furthermore, it explores the relationship between its microstructure and corrosion resistance in a 3.5%NaCl (mass fraction) solution. The results revealed that the LS treatment refines the martensite block size from 2.1 μm to 943 nm and increases the reversed austenite content from 1.9% to 7.8% compared with the HS treatment. The CLS treatment introduces a high density of dislocations and retained austenite, which provide favorable nucleation sites and diffusion pathways for elemental redistribution during aging, thereby leading to a substantial increase in reversed austenite content to 33%. Cyclic infiltration corrosion tests and electrochemical measurements confirm that grain refinement and the enhanced reversed austenite content considerably improve the corrosion resistance. Grain refinement increases the density of grain boundaries, facilitates the formation of a passivation film on the surface and reduces susceptibility to intergranular corrosion. Compared with LS-treated steel, CLS-treated steel exhibits a 109 mV_SCE increase in corrosion potential, an 86.25 μA/cm² decrease in corrosion current density, and a 26.82 mV_SCE increase in pitting potential. As the reversed austenite content increases, the total resistance of solution and the passivation film thickness increase, thereby improving the stability and protective performance of the passivation film. Concurrently, pitting charge transfer resistance increases, which improves resistance to pitting corrosion.