This study examines why and when proactive employees share knowledge. By integrating the Motivation-Opportunity-Ability Framework and Trait Activation Theory, and incorporating Mindsponge Theory, our multi-level model proposed that job autonomy moderates the impact of proactive personality on knowledge sharing (KS) within and between teams. Transformational leadership exhibits a cross-level effect on job autonomy. Utilizing a two-source, three-time-point research design, we collected data from 63 team leaders and 241 team members across six Chinese companies. Multilevel regression analysis revealed that within teams, increased job autonomy coupled with a proactive personality significantly enhanced KS. Between teams, job autonomy had a positive moderating effect. When job autonomy was low, more proactive teams exhibited less KS, whereas this negative effect was mitigated when job autonomy was high. The cross-level effect of transformational leadership on job autonomy was demonstrated. The theoretical and practical implications of these findings are discussed. •Job autonomy positively moderates proactivity-knowledge sharing intra and inter teams.•Transformational leadership has a positive cross-level effect on job autonomy.•Ability and opportunity factors are addressed in explaining proactivity's impacts.•Multi-level approach enhances insight into proactivity-knowledge sharing.