How to Improve the Corporate Sustainable Development?—The Importance of the Intellectual Capital and the Role of the Investor Confidence

In the era of the intelligent economy, to achieve business development, companies have shifted from focusing on physical resources to intangible resources and increasingly rely on investment in intangible assets. This phenomenon is a consensus. Intellectual capital is a skill that is difficult for competitors to replicate and is the most significant intangible asset of modern firms. Research on intellectual capital mainly focuses on the impact of companies’ performance; however, this study focuses on the sustainable development effects of companies. Taking manufacturing companies from Chinese A-share listings from 2015 to 2020 as the sample space and adding innovation capital based on the traditional intellectual capital model, a two-way fixed-effects model was used to conduct empirical research on the relationship between companies’ intellectual capital and corporate sustainable development. Meanwhile, research was conducted to test whether investor confidence innovatively played an intermediary role. This study showed that intellectual capital was positively correlated with the sustainable development of companies and investor confidence. In general, investor confidence was a bridge that connected intellectual capital and sustainable development. The results provided an empirical basis for companies that can enhance their sustainable development capabilities through intellectual capital investment. Managers should focus on the development of activities that have a positive impact on companies’ intellectual capital, increase the investment of intellectual capital, and achieve high-quality, sustainable development.