Online Unethical Behavior has increasingly become a prominent issue in the digital age. As a significant group of internet users, the ethical development of college students warrants attention. This study aims to develop a scale for measuring college students’ online Unethical Behavior to provide an assessment tool for online ethics education. The initial item pool for the scale was constructed using literature review and interviews, resulting in a preliminary questionnaire with 55 items across four dimensions: cyberbullying, online infringement, online deception, and online pornography. Through item analysis and exploratory factor analysis of survey data from 412 college students, a pilot questionnaire with 16 items across the same four dimensions was developed. Subsequently, confirmatory factor analysis was conducted with a sample of 532 college students to test the structural validity of the questionnaire, and reliability was also examined. The results indicate that the scale has good reliability and validity, comprising 16 items divided into four dimensions: cyberbullying, online infringement, online deception, and online pornography. This scale can be used to assess college students’ online Unethical Behavior and provide a basis for online ethics education in higher education institutions.