Background: STEAM education emphasizes transdisciplinary application and practice; however, there is a lack of comprehensive assessment tools for evaluating K-12 students' transdisciplinary learning. Given the cross-domain instructional and learning approaches in STEAM education, this study aims to develop and validate the Learn, Use, Practice, Design, Apply (LUPDA) as a comprehensive model for evaluating K-12 STEAM learning through rubrics-based assessment. Grounded in transdisciplinary theory, we propose five dimensions and 15 indicators, which are rigorously validated through confirmatory factor analysis (CFA) to ensure the reliability and structural integrity of the LUPDA model. Results: The results of the CFA demonstrated the following--(1) the LUPDA model satisfied both the univariate and multivariate normality distribution; (2) there are no offending estimates for the model's indicators; (3) 15 fit indices, including the absolute fit index, incremental fit index, and parsimonious fit index, all reached acceptable levels, confirming an excellent model fit; (4) the standardized factor loadings of all indicators exceeded 0.5, the average variance extracted (AVE) was greater than 0.5, and both Cronbach's a and composite reliability were above 0.7, indicating strong convergent validity; (5) the square root of the AVE for each dimension was significantly higher than the correlation coefficients between dimensions, confirming the discriminant validity of the LUPDA model. Conclusions: As an assessment tool for transdisciplinary STEAM education, this study focuses on developing and validating the structure, dimensions, and indicators of the LUPDA model. It provides a comprehensive assessment tool for K-12 STEAM education, offering a solution for effectively measuring students' transdisciplinary learning activities.