<p>In the context of sustainable development, environmental responsibility behavior in megaprojects has become critical. However, the comprehensive interplay of internal and external factors influencing contractors’ environmental responsibility behavior remains underexplored. This study integrates institutional theory and the theory of planned behavior into the Stimulus-Organism-Response framework to construct a theoretical model. Through this framework, the study investigates the driving factors and their mechanisms affecting the environmental responsibility behavior of megaproject contractors. A total of 256 questionnaires were collected from megaproject contractors, and partial least squares structural equation modeling was used for analysis. Results show that coercive, mimetic, and normative pressures positively influence contractors’ attitudes, subjective norms, and perceived behavioral control. These internal factors, in turn, positively affect contractors’ environmentally responsible behavior intention and ultimately their actual behavior. Furthermore, coercive and mimetic pressures positively impact contractors’ environmental responsibility behavior, and this relationship is enhanced by corporate green culture. Normative pressure also positively affects environmental responsibility behavior, with this effect strengthened by a strong moral identity. These findings provide theoretical and practical guidance for enhancing contractors’ environmental awareness and promoting sustainable development of megaprojects.</p>