Probing children's understanding of science concepts has been a focus of research in science education since the 1980s. In the process of learning, learners often produce many conceptions that differ from established scientific knowledge, which are commonly labelled as alternative conceptions (ACs). Knowing these ACs, their prevalence, nature, strength, causes and its dynamic change in the learning process is very important for the formation of scientific conceptions. In order to better identify middle school students' ACs at different times in mainland China and analyse the differences between these ACs, taking the concept of 'the human blood circulatory system' as an example, this study constructed and validated two sets of Four-Tier Multiple-Choice tests (4TMC), and applied them to explore 501 seventh graders after they had learned the concept. The results show that the 4TMC can effectively explore not only the individual ACs but also the group ACs. In the first test, 1 week after learning human blood circulatory system, six ACs have been identified in this group. There are five ACs in the second test 6 months after learning this knowledge. Meanwhile, there is a significant difference between these ACs at different times. This research can provide examples for exploring students' ACs.