The aim of this study was to explore the impact of a new solfege course design, where movable numbers integrated into the fixed-do system, on students' sight-singing skills. The procedure of this action research, carried out in two cycles, consists of three steps: 'planning', 'acting and observing' and 'reflecting'. The course, which was designed in the planning step, was implemented by the researcher for 28 weeks in the acting and observing step. Data were collected through research diary, student interviews, midterm and final exams, and student self-evaluations. In the reflecting step, evaluation regarding the entire process was made and an improved version of the course design was formed at the end of each cycle. According to the results of Cycle 1, the necessity of rearranging the tonal patterns, and progressing more slowly in rhythm teaching has emerged. The results of Cycle 2 revealed the need to update the 'rhythmic melody sight-singing' materials in the last unit and to incorporate video assignments into the course design. The overall results showed that integrating movable numbers into the fixed-do system is an effective way to teach music reading skills in solfege class.