According to the dual identities of affiliated firms of business groups，we analyze the dual attributes of the legitimacy and put forward the concept of ambidexterity of legitimacy． We divide the concept into two dimensions: balanced legitimacy and combined legitimacy，based on which we analyze the relationship between the ambidexterity of legitimacy and growth of affiliated firms． The result shows resource acquisition plays a significant mediation role in the relationship between the combined dimension and growth of affiliated firms，while the relationship between the balanced dimension and growth of affiliated firms is not supported． We also identify two moderating factors: environmental uncertainty and environmental munificence． The empirical study reveals that environmental uncertainty plays a significant positive moderating role in the relationship between the combines dimension and resource acquisition，while it plays a negative moderating role in the relationship between the balanced dimension and resource acquisition． However，environmental munificence plays a negative moderating role in the relationship between the combined dimension and resource acquisition，while it plays a positive moderating role in the relationship between balanced dimension and resource acquisition