This study focused on AOD stainless steel slag (AOD slag), utilizing a direct wet carbonation process at ambient temperature and pressure to effectively capture CO2. We first evaluated the impact of four distinct additives such as NaCl (Simulated seawater concentration, 2.43 mol/L), NaOH (Simulated cold rolling wastewater, 0.01 mol/L), NH4Cl (0.5 mol/L) and CH3COONH4 (0.5 mol/L) on the carbonation of AOD slag, ultimately selecting CH3COONH4 for its superior performance. Within the “AOD slag + CH3COONH4” system, we conducted a comprehensive examination of carbonation parameters, including additive concentration, particle size, solid-liquid ratio, temperature and time, with a focus on optimizing carbonation efficiency (ηc) and carbonation capacity (Cc). Additionally, we provided an in-depth exploration of the simultaneous efficient carbonation and detoxification mechanisms pertaining to the stainless steel slag. Under standard conditions (293 K, 1 atm), the system achieved an impressive ηc of 80.02%, and Cc of 386.62 kg CO2/ton slag, higher than those previously reported. Furthermore, after five experimental cycles, the system maintained a 70% original efficiency, with Cr leaching concentrations significantly below the national standard requirement of 1.5 mg/L. The in-situ treatment of slag and CO2 in the “AOD slag + CH3COONH4″ system is achieved through direct wet carbonation under environmental conditions. Its ultra-high carbonation efficiency and ability, as well as good cycling and non-toxic properties, demonstrate its potential for industrial application. Display omitted •An efficient direct carbonating AOD stainless steel slag method was proposed.•Improved CO2 fixation and detoxification of slag at ambient condition were realized.•The additive CH3COONH4 has great influence on carbonation performance of the slag.•Direct carbonation mechanism of “CH3COONH4+AOD slag” system was elucidated.•The detoxified slag powder meet the requirements of resource utilization.