Maraging stainless steels are the most widely used high strength stainless steels because of their excellent combination of high strength, superior corrosion resistance and good weldability. The typical heat treatment of maraging stainless steel consists of solution treatment and the following aging treatment. Aging treatment is the important process since it affects the steel's final properties and then determines its application prospect. Thus, understanding well the segregation behavior of alloying elements during the aging treatment plays a key role in developing the new maraging stainless steel with superior properties. In this work, segregation of alloying elements as well as its effect on mechanical properties and corrosion resistance of a multi-phase strengthened maraging stainless steel was studied by HRTEM and APT analyses. It was found that three precipitating species including Mo-rich R′ phase, η phase and Cr-rich α′ phase were identified in the steel. A unique core-shell structure with membrane-like R′ phase formed on the surface of η phase was identified however α′ phase distributed in the matrix separately. The core-shell structure enabled the maraging stainless steel a superior over-aging resistance and since aging time has reached 40 h, the characteristics of precipitations change little even aging time prolongs to 100 h. The corrosion test results indicated that the occurrence of α′ phase resulted in the formation of Cr-depleted zone and deteriorated the corrosion resistance seriously. In conclusion, the segregation behavior of alloying elements in maraging stainless steel has a significant effect on both mechanical property and corrosion resistance although some underlying mechanisms still haven't been understood well.