Based on synergistic governance of common institutional ownership, collusion fraud and their overlapping effects, this paper analyzes the impact of common institutional ownership on corporate bond financing costs and its mechanism by taking the data of China A-share non-financial listed companies from 2008 to 2023 as the research object. The results show that: common institutional ownership reduces the financing cost of corporate bonds by reducing agency costs, improving corporate information transparency and improving bond credit rating, which confirms the synergistic governance effect of common institutional ownership. Further analysis shows that the effect of common institutional ownership on reducing corporate bond financing costs is more significant in enterprises with high operational risk, severe financing constraints and high economic policy uncertainty, and common institutional ownership reduces bond financing costs and improves corporate performance. The above research conclusions not only enrich the existing research on the influencing factors of corporate bond financing costs, but also provide theoretical and practical basis for guiding the active participation of common institutional ownership in the governance of the bond market.