This study aimed to develop and validate a measurement tool for General Anxiety Disorder (GAD) and explore the underlying mechanisms of anxiety. Study 1 developed the Generalized Anxiety Symptom Scale (GASS) based on the diagnostic criteria of GAD in ICD-11, and tested its reliability and validity among secondary vocational students. The results showed that GASS had excellent internal consistency (Cronbach's α = 0.96), and unidimensional factor structure (81.60% variance explained). Criterion validity was supported by significant correlations with the GAD-7 (r = 0.69), and measurement invariance was confirmed across gender and over time. Based on three waves of longitudinal data, Study 2 explored the mechanisms underlying the effects of school climate, stress mindset, and self-compassion on anxiety among secondary vocational students. The results indicated that stress mindset played a partial mediating role between school climate and anxiety. Furthermore, moderated mediation analysis revealed that self-compassion moderated the relationship between stress mindset and anxiety. These findings demonstrate the robust psychometric properties of the GASS as a screening tool for GAD and highlight the interplay between environmental factors, such as school climate, and individual differences, such as self-compassion, in shaping anxiety outcomes, offering valuable insights for both assessment and intervention strategies.