“Investor primacy” serves as the cornerstone for the high-quality development of capital markets. However, under the development model characterized by scale expansion and traffic orientation, trading homogeneity among financial institutions has become increasingly severe. This paper innovatively examines the impact and micro-level mechanism of institutional trading homogeneity on the tail systemic risks of individual stocks from the perspective of trading horizon similarity. The findings reveal that trading homogeneity significantly increases the left-tail systemic risks of individual stocks. Mechanism testing indicates that liquidity shocks and buy-sell imbalance are important channels through which trading homogeneity strengthens the risk linkage between individual stocks and the market. Improving information disclosure quality and enhancing market confidence can effectively mitigate the risk effect of homogeneous trading. Further analysis shows that herd behavior in institutional research and speculation in concept stocks are significant drivers attracting homogeneous trading among similar investors. Trading homogeneity exacerbates risks under conditions of high policy uncertainty, bearish market trends, and elevated arbitrage risks. This paper provides a theoretical basis and policy implications for regulatory authorities to guide the differentiated development of financial institutions and prevent systemic risks, expands the micro foundation of financial health research, and has practical significance for maintaining financial market stability and enhancing the reliance of the financial system.