With the increasingly stringent global restrictions on carbon emissions, firms are facing growing carbon risks, which may not only drive up firms' pollution control costs but also force them to improve investment efficiency by optimizing investment and saving costs. Taking the Paris Agreement event as the background and using the data of listed companies in China from 2007 to 2020 and a difference-in-differences model, this paper reveals that the increase in carbon risk can significantly improve the investment efficiency of China's carbon-intensive firms. In addition, we find that the increase in carbon risk can encourage firms to carry out mergers and acquisitions to improve investment efficiency. Our findings provide Chinese evidence for the effective link between carbon risk and firm investment efficiency and may serve as a decision-making reference for carbon-intensive enterprises to improve investment efficiency through merger and acquisition activities in the context of increasing carbon risk.