Pulp and paper companies are characterized by high-energy consumption and high-emission in production, placing a heavy burden on the environment. Exploring ways to improve their environmental performance is urgently needed. External coercive pressure from environmental law and internal proactive environmental strategy both enable companies to perform better, but little is known about the difference between them. To close this knowledge gap, our analysis is conducted on a case company in pulp and paper industry to compare the differential impacts of the New Environmental Protection Law of China and the proactive environmental strategy on its environmental performance. In addition, accurately evaluating the environmental performance of pulp and paper companies is essential while research on this topic is scant. Therefore, a novel environmental performance evaluation system for pulp and paper companies (EPESPP) is constructed, including not only internal but also external evaluation indexes based on news reports of various media measured by SNOWNLP sentiment analysis. According to the system, the environmental performance of the case company from 2010 to 2018 is evaluated by the entropy-weight-based catastrophe progression method (EWCPM). The results of our case study show that the implementation of a proactive environmental strategy is more conducive to comprehensively improving corporate environmental performance. In other words, the driving effect of the New Environmental Protection Law is not as strong as that of the proactive environmental strategy.