Abstract: According to financial principal, enterprises should choose the investment projects with positive net present value and largest marginal profit. But in actual investment decision-making process, enterprises should consider more factors (e.g. customer relationship). Based on the data of Chinese A-share listed firms from 2008 to 2013, the empirical result shows that customer concentration is negatively correlated with enterprises’ investment efficiency. Further study found that the reason for the result is that the customer concentration leads to over investment, rather than under investment. Moreover, the relationship between customer concentration and investment efficiency only exist when the environment uncertainty is relatively low. There is no evidence shows the same effect can be found when environment uncertainty is relatively high.