Waste threatens human health and the environment. How can we persuade people to participate in waste separation? In order to address this challenge, the present experimental study (N=280) investigated the effects of perceived importance (high, medium, low) and language intensity (assertive, non-assertive) on people's intention to separate waste based on the social influence theory and the value-identity-personal norm model. The results showed that high perceived importance and assertive language were positively and significantly associated with waste separation intention. Furthermore, the mediating analysis revealed that environmental self-identity and personal norm were serial mediators in the relationship between perceived importance and waste separation intention. Therefore, strengthening perceived importance and enhancing internalization processes (environmental self-identity and personal norm) contribute to promoting waste separation intention. The findings of this study provide both theoretical and practical contributions to promote waste separation.