Objective To explore the effects of the case teaching based on Tanner model on clinical judgment and reasoning abilities of nursing undergraduates. Methods Eighty nursing undergraduates were recruited and divided into 2 online classes,and each class was divided into 8 groups. The online teaching focusing on geriatric cases was provided to these students. Students were guided to carry out inter-professional discussions based on the Tanner model and to do clinical judgement from the following dimensions of noticing,interpreting,responding,and reflecting. Before and after the course,students were asked to complete the questionnaires of the Chinese version of the Lasater Clinical Judgment Rubric（C-LCTR） and Self-Assessment of Clinical Reflection and Reasoning. The teaching feedback questionnaires were used to understand nursing students' evaluation on the teaching activities. Besides,students' changes and their suggestions for the teaching were also collected. Results After the course,the scores of clinical judgment of the nursing undergraduates were higher than those before the course in noticing,interpreting,responding and reflecting dimensions and the total C-LCJR score（P<0.001）. The scores of Self-Assessment of Clinical Reflection and Reasoning of nursing undergraduates were also higher compared to those before the course（P<0.001）. A majority of nursing undergraduates agreed that they would be more confident to be engaged in geriatric health care in the future. They were satisfied with the teaching activities as a whole,and thought that the learning objectives of the curriculum were in line with the needs of ability development. Conclusion The multidisciplinary online case teaching based on Tanner model can improve the clinical judgment ability and clinical reasoning of nursing undergraduates. The course is based on the cases of the older adults with multiple diseases and it can enhance students' confidence to be involved in geriatric nursing.