This study developed and validated the Human–Computer Collaborative Classroom Second Language Learning Engagement Scale among 710 junior high school students studying in Mongolian. Initially, the scale’s conceptual framework was developed through a review of pertinent literature and interview, drawing on self-determination theory and socio-constructivist perspectives to define engagement in human–computer collaborative second language learning contexts. The study adopted a sequential mixed-methods design: in Phase 1, item analysis and exploratory factor analysis (EFA) were conducted using data from 437 students, resulting in a preliminary five-factor structure; in Phase 2, confirmatory factor analysis (CFA) was performed using data from the remaining 273 students to validate the factor structure. The final scale comprises five core dimensions: (1) higher-order thinking, (2) student–teacher interaction, (3) human–computer interaction, (4) active collaborative learning, and (5) learning enthusiasm. Structural equation modeling confirmed a robust five-factor model, with all fit indices indicating satisfactory model fit (e.g., CFI = 0.981, TLI = 0.977, RMSEA = 0.041). The scale demonstrates strong internal consistency (Cronbach’s α = 0.959) and construct validity. These findings highlight the reliability and efficacy of this psychometric tool for evaluating students’ engagement in second language learning within human–computer collaborative classroom environments, offering valuable insights for educators and researchers.