The investment efficiency of company directly affects the company to realize the goal of maximizing value. Recently, it is common to see inefficient investment in companies of China. Whether the corporate governance mechanism is sound or not affects the investment efficiency of company. Can internal audit, as one of the four cornerstones of corporate governance, affect company’s investment efficiency? Based on this, the paper makes an empirical study on whether internal audit is able to reduce the inefficient investment and improve the investment efficiency in a company. Study results have shown that there exists a negative connection between the quality of internal audit and overinvestment or underinvestment. The research has enriched the related reviews of internal audit and investment efficiency. Meanwhile, it has also put forward a new approach to enhancing investment efficiency.