Online tests have been identified as a core learning activity in higher education. Unlike conventional online tests, which cannot completely reflect students' learning status, two-tier tests not only consider students' answers, but also take into account reasons for their answers. Due to such significance, research into the two-tier tests had mushroomed but few studies examined why the two-tier test approach was effective. To this end, we conducted an empirical study, where a lag sequential analysis was used to analyze behavior patterns while qualitative data from the questionnaire were applied to explain why these behavior patterns were happened. The results indicated students with a two-tier test tended to realize the rationale of a concept, instead of relying on their memories. In other words, the two-tier test can facilitate students to develop deep thinking skills. This may be because they considered the two-tier test as a learning tool, instead of an assessment tool only.