Many educational and psychological studies have shown that the development of students is generally step-by-step (i.e. ordinal development) to a specific level. This study proposed a novel longitudinal learning diagnosis model with polytomous attributes to track students' ordinal development in learning. Using the concept of polytomous attributes in the proposed model, the learning process of a specific skill, from non-mastery to mastery, can be divided into multiple ordinal steps in order to better characterize the learning trajectory. The results of an empirical study conducted to explore the performance of the proposed model indicated that it could adequately diagnose the ordinal development of skills in longitudinal assessments. A simulation study was also conducted to examine the estimation accuracy of general ability and the classification accuracy of attributes of the proposed model in different simulated conditions.