This study aimed to validate a learning progression (LP) for the scientific imagination (Authors, 2015) based on a measurement approach using the BEAR Assessment System (BAS; Wilson, 2005, 2009) in an attempt to better understand the core ideas and developmental path of the scientific imagination process as well as align curriculum, instruction, and assessment through LP. Participants in this study were selected from Taiwan and classified into two categories. The first category, which included 767 third to sixth grade elementary school students, was administered the Scientific Imagination Test-Verbal (SIT-Verbal). The second category consisted of two award-winning teachers who were interviewed in order to achieve more information for the LP. The SIT-Verbal, Imagery Questionnaire, and the outline of interview were used in this study. Among these, the SIT-Verbal covered four key components of the scientific imagination process: brainstorming, association, transformation/elaboration, and conceptualization/organization/formation. The Imagery Questionnaire covered imagery ability, which could generate imagery, objects, and pictures in one’s mind. For analysis, the multiple validities (Wolfe & Smith, 2007) of the SIT-Verbal were assessed using the Rasch partial credit model. The findings showed that (1) the BEAR assessment system and Rasch measurement approaches provided a feasible framework for developing validated tools to assess the LP of scientific imagination; (2) the empirical data supported that students’ scientific imagination progressed “from brainstorming, association, and transformation/elaboration to conceptualization/organization/formation.” Implications for the assessments with the LPs and revisions for both the SIT-Verbal and the scientific imagination LP are also proposed.