Waste sorting is essential to address the current predicament of waste management. Though it is important, insufficient attention has been paid to explore residents’ waste sorting intention and behavior and understand its formation process. To narrow the research gap, this research built a theoretical research model by adding personal moral norms and waste sorting knowledge into the theory of planned behavior to explicate residents’ waste sorting intention and behavior formation process. Meanwhile, given the discrepancy between waste sorting intention and actual behavior, this research also explored the effect of external conditions, such as incentive measures, on this discrepancy. Based on survey data from 397 Chinese residents, this research found that attitudes, subjective norms, perceived behavioral control, personal moral norms and waste sorting knowledge were directly and significantly related to residents’ waste sorting intention. Waste sorting knowledge also had an indirect influence on residents’ waste sorting intention through attitudes and perceived behavioral control. Additionally, this research corroborated the discrepancy between waste sorting intention and behavior, and suggested that the link between intention and behavior was contingent on incentive measures. Incentive measures strengthened the effect of intention on behavior. This research is useful for understanding residents’ waste sorting intention and behavior and valuable for encouraging residents to sort waste in their daily lives.