This study explores the dynamic strain aging mechanism in medium-Mn steel, emphasizing its interplay with the kinetics of strain-induced martensite transformation (SIMT). By tracking the migration of each Portevine-Le Châtelier band, we investigated the complex dynamics of SIMT and its impact on the mechanical behavior. Characteristics of stress serrations are closely related to the dynamic propagation behavior of PLC bands. The ε martensite, being softer than austenite, contributes negligibly to the work hardening rate, while plays a key role in coordinating plastic deformation. The α′ martensite with the hardest value of 6.08 GPa, locally pins the mobile dislocations, triggering the stress serrations in the stress-strain curve. However, as α′ martensite grows, the pinning effect on dislocations diminishes substantially. We delimited a critical size of α′ martensite, about 350 nm, beyond which it loses the ability to pin dislocations and instead begins to undergo plastic deformation. Therefore, the serrations intensity is closely governed by the nucleation and growth rates of α′ martensite: a nucleation-dominant regime intensifies serrations, while a growth-dominant regime reduces them. The deep learning of the effect of SIMT kinetics on DSA mechanism is pivotal for understanding the mechanical stability and ductility of medium-Mn steels under strain.