Waste threatens human health and the environment. How can we persuade people to participate in waste separation? In order to address this challenge, we conducted an experimental study (N = 280) in which we manipulated perceived importance (high, medium, low) and language intensity (assertive, non-assertive) to investigate their effects on people's intention to separate waste. 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 of 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.