The purpose of this study is to explore how prior knowledge influences thinking strategies and patterns of abductive reasoning during the experimental design process in life science contexts. The participants were 23 pre-service elementary teachers (6 male, 17 female) from a teacher education university. To examine the influence of prior knowledge, two tasks were employed: a familiar task based on the curriculum where participants possessed prior knowledge, and an unfamiliar task lacking direct experience. Participants completed the tasks sequentially, and their design artifacts and post-interview data were comprehensively analyzed. The results are as follows. First, while the experimental design process followed a five-stage reasoning flow, fundamental approach strategies differed based on task familiarity and predictability. In the high prior knowledge task, participants primarily utilized a ‘hypothesis-deductive approach,’ systematically controlling variables based on existing knowledge to perform ‘scientific design.’ Conversely, in the unfamiliar task where outcome prediction was difficult, an ‘exploratory approach’ emerged. Participants relied on intuition and experience to construct verification logic, shifting towards ‘engineering design’ with the practical goal of realizing observable phenomena. Second, this paradigm shift was accompanied by a change in the functional role of abductive reasoning. Under ‘scientific design,’ ‘selective abduction’ mainly performed a complementary function to stabilize the design by supplementing existing frameworks. However, in the transition to ‘engineering design,’ ‘creative abduction’ was actively utilized for its generative function to overcome uncertainty and devise new experimental conditions. This study is significant as it reveals that learners flexibly switch design paradigms in response to uncertainty and strategically adapt their use of abductive reasoning, thereby providing a new framework for understanding adaptive thinking strategies.