This paper selects listed chemical industry enterprises in Shanghai and Shenzhen A-shares from 2010-2019 as a research sample, and innovatively explores the relationship between environmental protection tax, environmental protection investment and corporate green technological innovation and the corresponding mechanism of action using causal intermediary effect analysis and threshold regression model. It is found that: environmental protection tax inhibits green technology innovation of chemical companies in China, and environmental protection investment plays a mediating role between environmental protection tax and green technology innovation of companies, and the mediating effect of environmental protection investment is 2.35%. There is a single “threshold effect” between environmental protection tax and environmental protection investment, which inhibits environmental protection investment both before and after the threshold, and the inhibiting effect decreases after the threshold is crossed. The current environmental protection tax rate in China’s chemical industry is still at the left end of the threshold, and the inhibiting effect on environmental protection investment is still at a strong stage. To improve the green technology innovation of chemical enterprises in the future, we need to improve the environmental protection tax system, raise the awareness of enterprises themselves and develop corresponding incentive supporting policies.