In scientific journals published all over the world, the economic performance of firms with ISO 14001 certification is a highly controversial topic. This controversy is particularly evident in developing countries. With regard to this topic, we maintain a neutral position in this paper. Using the data of 63 firms from the Shenzhen and Shanghai Stock Exchanges in China during the period of 2003–2018, and employing the difference-in-difference method, we analyze the influence of ISO 14001 certification on firms’ financial performance. We also examine the moderating effect of firm size and age, considering the time factor. In addition, we test the influence of ISO 14001 certification on exports, using China’s provincial data from 2008 to 2018. The results indicate that ISO 14001 certification negatively affects the financial performance of firms; also, the negative effects decrease in line with the increase of certification time. Firm size and age are positively related to the relationship between ISO 14001 certification and a firm’s financial performance. At the same time, ISO 14001 certification can effectively improve a firm’s competitiveness in the international market. The study combines two opposing positions found in existing literature regarding the effectiveness of ISO 14001 certification. The results show that ISO 14001 certification will ultimately have a positive impact on China’s economy, but that influence should be correctly evaluated in terms of the effect on small and medium-sized enterprises. Some suggestions for firm managers and the government are then discussed.